diff --git a/.github/workflows/nnbd-test.yaml b/.github/workflows/nnbd-test.yaml
new file mode 100644
index 0000000000..fdeaaaf0e0
--- /dev/null
+++ b/.github/workflows/nnbd-test.yaml
@@ -0,0 +1,70 @@
+name: Test
+
+on:
+  # Run CI on pushes to the main branch, and on PRs against main.
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+  schedule:
+    - cron: 0 15 * * *
+
+env:
+  PUB_ENVIRONMENT: bot.github
+
+jobs:
+  test:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest]
+        sdk: [dev, stable]
+        job: [main, flutter, sdk-analyzer, packages, sdk-docs]
+        include:
+          - os: macos-latest
+            sdk: dev
+            job: main
+          - os: windows-latest
+            sdk: dev
+            job: main
+        exclude:
+            # Do not try to run flutter against the "stable" sdk,
+            # it is unlikely to work and produces uninteresting
+            # results.
+          - sdk: stable
+            job: flutter
+          - sdk: stable
+            job: sdk-analyzer
+
+    steps:
+      - name: Configure git
+        if: runner.os == 'Windows'
+        run: git config --global core.autocrlf input
+      - uses: actions/checkout@v2
+      - uses: dart-lang/setup-dart@v1.0
+        with:
+          sdk: ${{ matrix.sdk }}
+      - name: Install dependencies
+        run: dart pub get
+      - name: ${{ matrix.job }}
+        if: runner.os != 'Windows'
+        run: ./tool/travis.sh
+        env:
+          DARTDOC_BOT: ${{ matrix.job }}
+          #COVERAGE_TOKEN: true # this needs to be set to enable coverage
+      - name: ${{ matrix.job }}
+        if: runner.os == 'Windows' && matrix.job == 'main'
+        run: dart run grinder buildbot
+        env:
+          DARTDOC_BOT: ${{ matrix.job }}
+     # - id: coverage
+     #   name: Upload coverage
+     #   if: runner.os == 'Linux' && matrix.job == 'main' && matrix.sdk == 'dev'
+     #   uses: coverallsapp/github-action@v1.1.2
+     #   with:
+     #     github-token: ${{ secrets.GITHUB_TOKEN }}
+     #     path-to-lcov: lcov.info
+     # - name: Echo coveralls api result
+     #   if: runner.os == 'Linux' && matrix.job == 'main' && matrix.sdk == 'dev'
+     #   run: echo ${{ steps.coverage.outputs['coveralls-api-result'] }}
diff --git a/.github/workflows/test.yaml b/.github/workflows/test.yaml
index fdeaaaf0e0..4f3f3c50c9 100644
--- a/.github/workflows/test.yaml
+++ b/.github/workflows/test.yaml
@@ -1,11 +1,11 @@
-name: Test
+name: Test - NNBD branch
 
 on:
   # Run CI on pushes to the main branch, and on PRs against main.
   push:
-    branches: [ master ]
+    branches: [ nnbd ]
   pull_request:
-    branches: [ master ]
+    branches: [ nnbd ]
   schedule:
     - cron: 0 15 * * *
 
@@ -19,23 +19,15 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        sdk: [dev, stable]
-        job: [main, flutter, sdk-analyzer, packages, sdk-docs]
+        sdk: [dev]
+        job: [nnbd, flutter, sdk-analyzer, packages, sdk-docs]
         include:
           - os: macos-latest
             sdk: dev
-            job: main
+            job: nnbd
           - os: windows-latest
             sdk: dev
-            job: main
-        exclude:
-            # Do not try to run flutter against the "stable" sdk,
-            # it is unlikely to work and produces uninteresting
-            # results.
-          - sdk: stable
-            job: flutter
-          - sdk: stable
-            job: sdk-analyzer
+            job: nnbd
 
     steps:
       - name: Configure git
@@ -54,8 +46,8 @@ jobs:
           DARTDOC_BOT: ${{ matrix.job }}
           #COVERAGE_TOKEN: true # this needs to be set to enable coverage
       - name: ${{ matrix.job }}
-        if: runner.os == 'Windows' && matrix.job == 'main'
-        run: dart run grinder buildbot
+        if: runner.os == 'Windows' && matrix.job == 'nnbd'
+        run: dart run grinder buildbot-no-publish
         env:
           DARTDOC_BOT: ${{ matrix.job }}
      # - id: coverage
diff --git a/bin/dartdoc.dart b/bin/dartdoc.dart
index 83b3f6180f..9b9b87a961 100644
--- a/bin/dartdoc.dart
+++ b/bin/dartdoc.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.bin;
 
 import 'dart:async';
diff --git a/lib/dartdoc.dart b/lib/dartdoc.dart
index 70c31c775d..c4a8649a33 100644
--- a/lib/dartdoc.dart
+++ b/lib/dartdoc.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 /// A documentation generator for Dart.
 ///
 /// Library interface is currently under heavy construction and may change
diff --git a/lib/options.dart b/lib/options.dart
index 537c83084f..3a39c4e8fe 100644
--- a/lib/options.dart
+++ b/lib/options.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'dart:io' show stderr, exitCode;
 
 import 'package:analyzer/file_system/file_system.dart';
diff --git a/lib/src/comment_references/model_comment_reference.dart b/lib/src/comment_references/model_comment_reference.dart
index 7d6e0fcbfb..7416994650 100644
--- a/lib/src/comment_references/model_comment_reference.dart
+++ b/lib/src/comment_references/model_comment_reference.dart
@@ -2,6 +2,9 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
+
+// @dart=2.9
+
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/file_system/file_system.dart';
diff --git a/lib/src/comment_references/parser.dart b/lib/src/comment_references/parser.dart
index 6b09ccd308..37d507bd0e 100644
--- a/lib/src/comment_references/parser.dart
+++ b/lib/src/comment_references/parser.dart
@@ -2,6 +2,9 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
+
+// @dart=2.9
+
 import 'package:charcode/charcode.dart';
 import 'package:meta/meta.dart';
 
diff --git a/lib/src/dartdoc_options.dart b/lib/src/dartdoc_options.dart
index b8eba96f65..00ae7448c7 100644
--- a/lib/src/dartdoc_options.dart
+++ b/lib/src/dartdoc_options.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 ///
 /// dartdoc's dartdoc_options.yaml configuration file follows similar loading
 /// semantics to that of analysis_options.yaml,
diff --git a/lib/src/element_type.dart b/lib/src/element_type.dart
index b56dca3336..95f0429db2 100644
--- a/lib/src/element_type.dart
+++ b/lib/src/element_type.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 /// The models used to represent Dart code.
 library dartdoc.element_type;
 
diff --git a/lib/src/experiment_options.dart b/lib/src/experiment_options.dart
index 017dc942f3..5bf0edd84a 100644
--- a/lib/src/experiment_options.dart
+++ b/lib/src/experiment_options.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 ///
 /// Implementation of Dart language experiment option handling for dartdoc.
 /// See https://github.com/dart-lang/sdk/blob/master/docs/process/experimental-flags.md.
diff --git a/lib/src/generator/dartdoc_generator_backend.dart b/lib/src/generator/dartdoc_generator_backend.dart
index 8b2d6aae8c..44967238da 100644
--- a/lib/src/generator/dartdoc_generator_backend.dart
+++ b/lib/src/generator/dartdoc_generator_backend.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:dartdoc/dartdoc.dart';
 import 'package:dartdoc/options.dart';
diff --git a/lib/src/generator/empty_generator.dart b/lib/src/generator/empty_generator.dart
index bd2dbb2cfb..4962848af5 100644
--- a/lib/src/generator/empty_generator.dart
+++ b/lib/src/generator/empty_generator.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 library dartdoc.empty_generator;
 
 import 'package:dartdoc/src/dartdoc_options.dart';
diff --git a/lib/src/generator/generator.dart b/lib/src/generator/generator.dart
index 3253d60071..2a5c2f319e 100644
--- a/lib/src/generator/generator.dart
+++ b/lib/src/generator/generator.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 /// A library containing an abstract documentation generator.
 library dartdoc.generator;
 
diff --git a/lib/src/generator/generator_frontend.dart b/lib/src/generator/generator_frontend.dart
index 08d9f3caff..30e31ab184 100644
--- a/lib/src/generator/generator_frontend.dart
+++ b/lib/src/generator/generator_frontend.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/generator/generator.dart';
 import 'package:dartdoc/src/logging.dart';
 import 'package:dartdoc/src/model/model.dart';
diff --git a/lib/src/generator/generator_utils.dart b/lib/src/generator/generator_utils.dart
index 122e927605..83b7860b7b 100644
--- a/lib/src/generator/generator_utils.dart
+++ b/lib/src/generator/generator_utils.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:convert';
 
 import 'package:collection/collection.dart';
diff --git a/lib/src/generator/html_generator.dart b/lib/src/generator/html_generator.dart
index bf73bcc889..81f00dd9b9 100644
--- a/lib/src/generator/html_generator.dart
+++ b/lib/src/generator/html_generator.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.html_generator;
 
 import 'package:analyzer/file_system/file_system.dart';
diff --git a/lib/src/generator/markdown_generator.dart b/lib/src/generator/markdown_generator.dart
index e51e7b477b..d60d4c9735 100644
--- a/lib/src/generator/markdown_generator.dart
+++ b/lib/src/generator/markdown_generator.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:dartdoc/options.dart';
 import 'package:dartdoc/src/generator/dartdoc_generator_backend.dart';
diff --git a/lib/src/generator/resource_loader.dart b/lib/src/generator/resource_loader.dart
index b4f77842d3..aa5a53b4e3 100644
--- a/lib/src/generator/resource_loader.dart
+++ b/lib/src/generator/resource_loader.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 /// Make it possible to load resources from the dartdoc code repository.
 library dartdoc.resource_loader;
 
diff --git a/lib/src/generator/template_data.dart b/lib/src/generator/template_data.dart
index 594816fd50..e505359425 100644
--- a/lib/src/generator/template_data.dart
+++ b/lib/src/generator/template_data.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/model/model.dart';
 
 typedef ContainerSidebar = String Function(
diff --git a/lib/src/generator/templates.aot_renderers_for_html.dart b/lib/src/generator/templates.aot_renderers_for_html.dart
index 0ece2698bb..8eb4ee4fcd 100644
--- a/lib/src/generator/templates.aot_renderers_for_html.dart
+++ b/lib/src/generator/templates.aot_renderers_for_html.dart
@@ -7,6 +7,7 @@
 // the variable is not used; generally when the section is checking if a
 // non-bool, non-Iterable field is non-null.
 // ignore_for_file: unused_local_variable
+// @dart=2.9
 
 import 'dart:convert' as _i18;
 
diff --git a/lib/src/generator/templates.aot_renderers_for_md.dart b/lib/src/generator/templates.aot_renderers_for_md.dart
index 32f53245a2..81f508f7c0 100644
--- a/lib/src/generator/templates.aot_renderers_for_md.dart
+++ b/lib/src/generator/templates.aot_renderers_for_md.dart
@@ -7,6 +7,7 @@
 // the variable is not used; generally when the section is checking if a
 // non-bool, non-Iterable field is non-null.
 // ignore_for_file: unused_local_variable
+// @dart=2.9
 
 import 'dart:convert' as _i18;
 
diff --git a/lib/src/generator/templates.dart b/lib/src/generator/templates.dart
index 7d99dbf768..ae57a18a50 100644
--- a/lib/src/generator/templates.dart
+++ b/lib/src/generator/templates.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 @Renderer(#renderCategory, Context<CategoryTemplateData>(), 'category',
     visibleTypes: _visibleTypes)
 @Renderer(#renderClass, Context<ClassTemplateData>(), 'class')
diff --git a/lib/src/generator/templates.runtime_renderers.dart b/lib/src/generator/templates.runtime_renderers.dart
index 1c0afd7366..476d4695a1 100644
--- a/lib/src/generator/templates.runtime_renderers.dart
+++ b/lib/src/generator/templates.runtime_renderers.dart
@@ -5,6 +5,7 @@
 
 // ignore_for_file: camel_case_types, deprecated_member_use_from_same_package
 // ignore_for_file: unused_import
+// @dart=2.9
 import 'package:dartdoc/dartdoc.dart';
 import 'package:dartdoc/src/generator/template_data.dart';
 import 'package:dartdoc/src/model/annotation.dart';
diff --git a/lib/src/io_utils.dart b/lib/src/io_utils.dart
index 2be1ffa625..97a95c18a6 100644
--- a/lib/src/io_utils.dart
+++ b/lib/src/io_utils.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 /// This is a helper library to make working with io easier.
 library dartdoc.io_utils;
 
diff --git a/lib/src/logging.dart b/lib/src/logging.dart
index b56a8d39c1..8c87b968c7 100644
--- a/lib/src/logging.dart
+++ b/lib/src/logging.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:convert';
 import 'dart:io' show stderr, stdout;
 
diff --git a/lib/src/markdown_processor.dart b/lib/src/markdown_processor.dart
index 74d7f9c2ea..08a02797c9 100644
--- a/lib/src/markdown_processor.dart
+++ b/lib/src/markdown_processor.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 /// Utility code to convert markdown comments to html.
 library dartdoc.markdown_processor;
 
diff --git a/lib/src/matching_link_result.dart b/lib/src/matching_link_result.dart
index 5cd1130916..f19d9b10f8 100644
--- a/lib/src/matching_link_result.dart
+++ b/lib/src/matching_link_result.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/element_type.dart';
 import 'package:dartdoc/src/model/comment_referable.dart';
 import 'package:dartdoc/src/model/model.dart';
diff --git a/lib/src/model/accessor.dart b/lib/src/model/accessor.dart
index 48daf92fdf..2d598809f9 100644
--- a/lib/src/model/accessor.dart
+++ b/lib/src/model/accessor.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/source/line_info.dart';
 import 'package:analyzer/src/dart/element/member.dart' show ExecutableMember;
diff --git a/lib/src/model/annotation.dart b/lib/src/model/annotation.dart
index 0e6bc696a6..d03a8a2835 100644
--- a/lib/src/model/annotation.dart
+++ b/lib/src/model/annotation.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:dartdoc/src/element_type.dart';
 import 'package:dartdoc/src/model/feature.dart';
diff --git a/lib/src/model/canonicalization.dart b/lib/src/model/canonicalization.dart
index b22a9e6e9c..ba6ccba6f8 100644
--- a/lib/src/model/canonicalization.dart
+++ b/lib/src/model/canonicalization.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/comment_references/model_comment_reference.dart';
 import 'package:dartdoc/src/model/model.dart';
 
diff --git a/lib/src/model/categorization.dart b/lib/src/model/categorization.dart
index 5652e1209f..f477773af1 100644
--- a/lib/src/model/categorization.dart
+++ b/lib/src/model/categorization.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/model/model.dart';
 
 final RegExp _categoryRegExp = RegExp(
diff --git a/lib/src/model/category.dart b/lib/src/model/category.dart
index 02e3d96a7a..a6c5c9916a 100644
--- a/lib/src/model/category.dart
+++ b/lib/src/model/category.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:dartdoc/src/comment_references/model_comment_reference.dart';
diff --git a/lib/src/model/class.dart b/lib/src/model/class.dart
index db3f826765..63e4c9c278 100644
--- a/lib/src/model/class.dart
+++ b/lib/src/model/class.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:dartdoc/src/element_type.dart';
diff --git a/lib/src/model/comment_referable.dart b/lib/src/model/comment_referable.dart
index d7696cfa0e..224b2409bd 100644
--- a/lib/src/model/comment_referable.dart
+++ b/lib/src/model/comment_referable.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 ///
 /// Code for managing comment reference lookups in dartdoc.
 ///
diff --git a/lib/src/model/constructor.dart b/lib/src/model/constructor.dart
index 9a01ea5871..66b22317bf 100644
--- a/lib/src/model/constructor.dart
+++ b/lib/src/model/constructor.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/source/line_info.dart';
 import 'package:dartdoc/src/element_type.dart';
diff --git a/lib/src/model/container.dart b/lib/src/model/container.dart
index 75becea76b..13adf614a5 100644
--- a/lib/src/model/container.dart
+++ b/lib/src/model/container.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/scope.dart';
 import 'package:dartdoc/src/model/comment_referable.dart';
diff --git a/lib/src/model/container_member.dart b/lib/src/model/container_member.dart
index 1898bc0ba0..f3ec632515 100644
--- a/lib/src/model/container_member.dart
+++ b/lib/src/model/container_member.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/model/feature.dart';
 import 'package:dartdoc/src/model/model.dart';
 import 'package:meta/meta.dart';
diff --git a/lib/src/model/documentable.dart b/lib/src/model/documentable.dart
index 4779b0bda6..dc773426b8 100644
--- a/lib/src/model/documentable.dart
+++ b/lib/src/model/documentable.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:dartdoc/src/dartdoc_options.dart';
 import 'package:dartdoc/src/io_utils.dart';
diff --git a/lib/src/model/documentation.dart b/lib/src/model/documentation.dart
index 44dc46d891..8830e78d2c 100644
--- a/lib/src/model/documentation.dart
+++ b/lib/src/model/documentation.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/comment_references/model_comment_reference.dart';
 import 'package:dartdoc/src/markdown_processor.dart';
 import 'package:dartdoc/src/model/model.dart';
diff --git a/lib/src/model/documentation_comment.dart b/lib/src/model/documentation_comment.dart
index 1422cc8f5b..ae8a5335ff 100644
--- a/lib/src/model/documentation_comment.dart
+++ b/lib/src/model/documentation_comment.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:args/args.dart';
 import 'package:crypto/crypto.dart' as crypto;
 import 'package:dartdoc/src/model/model.dart';
diff --git a/lib/src/model/dynamic.dart b/lib/src/model/dynamic.dart
index a345f4c8c6..4d130dd65e 100644
--- a/lib/src/model/dynamic.dart
+++ b/lib/src/model/dynamic.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:dartdoc/src/element_type.dart';
 import 'package:dartdoc/src/model/comment_referable.dart';
diff --git a/lib/src/model/enclosed_element.dart b/lib/src/model/enclosed_element.dart
index 228be4b30a..bc285f4505 100644
--- a/lib/src/model/enclosed_element.dart
+++ b/lib/src/model/enclosed_element.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/model/model.dart';
 
 /// An element that is enclosed by some other element.
diff --git a/lib/src/model/enum.dart b/lib/src/model/enum.dart
index c6c6da6f85..93762618cb 100644
--- a/lib/src/model/enum.dart
+++ b/lib/src/model/enum.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 // TODO(jcollins-g): Consider Enum as subclass of Container?
 import 'package:analyzer/dart/element/element.dart';
 import 'package:dartdoc/src/model/model.dart';
diff --git a/lib/src/model/extendable.dart b/lib/src/model/extendable.dart
index bb496c6b4f..909bd30a13 100644
--- a/lib/src/model/extendable.dart
+++ b/lib/src/model/extendable.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/model/model.dart';
 
 /// Mixin for subclasses of [ModelElement] representing Elements that can be
diff --git a/lib/src/model/extension.dart b/lib/src/model/extension.dart
index ff0704c5af..b189b42fc9 100644
--- a/lib/src/model/extension.dart
+++ b/lib/src/model/extension.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:dartdoc/src/element_type.dart';
 import 'package:dartdoc/src/model/comment_referable.dart';
diff --git a/lib/src/model/extension_target.dart b/lib/src/model/extension_target.dart
index bea6e7c65f..3291c2de05 100644
--- a/lib/src/model/extension_target.dart
+++ b/lib/src/model/extension_target.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/element_type.dart';
 import 'package:dartdoc/src/model/model.dart';
 
diff --git a/lib/src/model/feature.dart b/lib/src/model/feature.dart
index c899c09be1..eaa2638c8b 100644
--- a/lib/src/model/feature.dart
+++ b/lib/src/model/feature.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:collection/collection.dart';
 import 'package:dartdoc/src/model/privacy.dart';
 
diff --git a/lib/src/model/feature_set.dart b/lib/src/model/feature_set.dart
index f3bbb58daf..31b67d84cf 100644
--- a/lib/src/model/feature_set.dart
+++ b/lib/src/model/feature_set.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/model/language_feature.dart';
 import 'package:dartdoc/src/model/model.dart';
 
diff --git a/lib/src/model/field.dart b/lib/src/model/field.dart
index d259ff916d..0542828abe 100644
--- a/lib/src/model/field.dart
+++ b/lib/src/model/field.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:dartdoc/src/model/feature.dart';
diff --git a/lib/src/model/getter_setter_combo.dart b/lib/src/model/getter_setter_combo.dart
index 134cd6c045..53d353e53a 100644
--- a/lib/src/model/getter_setter_combo.dart
+++ b/lib/src/model/getter_setter_combo.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:convert';
 
 import 'package:analyzer/dart/ast/ast.dart'
diff --git a/lib/src/model/indexable.dart b/lib/src/model/indexable.dart
index 0e46d0d1fd..24d47da254 100644
--- a/lib/src/model/indexable.dart
+++ b/lib/src/model/indexable.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/model/model.dart';
 
 /// Something able to be indexed.
diff --git a/lib/src/model/inheritable.dart b/lib/src/model/inheritable.dart
index 5b6a9dd4cb..d3d01c25a1 100644
--- a/lib/src/model/inheritable.dart
+++ b/lib/src/model/inheritable.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/model/feature.dart';
 import 'package:dartdoc/src/model/model.dart';
 import 'package:dartdoc/src/special_elements.dart';
diff --git a/lib/src/model/language_feature.dart b/lib/src/model/language_feature.dart
index 9045675402..79f65ed7f2 100644
--- a/lib/src/model/language_feature.dart
+++ b/lib/src/model/language_feature.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/render/language_feature_renderer.dart';
 
 const Map<String, String> _featureDescriptions = {
diff --git a/lib/src/model/library.dart b/lib/src/model/library.dart
index 0d473879b0..42a11c933d 100644
--- a/lib/src/model/library.dart
+++ b/lib/src/model/library.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart' hide CommentReference;
diff --git a/lib/src/model/library_container.dart b/lib/src/model/library_container.dart
index 2a10757c4c..46c94593d5 100644
--- a/lib/src/model/library_container.dart
+++ b/lib/src/model/library_container.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:collection/collection.dart';
 import 'package:dartdoc/src/model/model.dart';
 import 'package:dartdoc/src/model_utils.dart' as model_utils;
diff --git a/lib/src/model/locatable.dart b/lib/src/model/locatable.dart
index f140fb90ca..94d03d1ae0 100644
--- a/lib/src/model/locatable.dart
+++ b/lib/src/model/locatable.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart' show Element;
 
 /// Something that can be located for warning purposes.
diff --git a/lib/src/model/method.dart b/lib/src/model/method.dart
index 05c6f12c3e..9873457d29 100644
--- a/lib/src/model/method.dart
+++ b/lib/src/model/method.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/source/line_info.dart';
 import 'package:analyzer/src/dart/element/member.dart' show ExecutableMember;
diff --git a/lib/src/model/mixin.dart b/lib/src/model/mixin.dart
index 6daaa12270..424c8b9fa2 100644
--- a/lib/src/model/mixin.dart
+++ b/lib/src/model/mixin.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:dartdoc/src/element_type.dart';
diff --git a/lib/src/model/model.dart b/lib/src/model/model.dart
index 34557bf9ef..9591fe6394 100644
--- a/lib/src/model/model.dart
+++ b/lib/src/model/model.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 export 'accessor.dart';
 export 'canonicalization.dart';
 export 'categorization.dart';
diff --git a/lib/src/model/model_element.dart b/lib/src/model/model_element.dart
index f01ce12963..f922194ceb 100644
--- a/lib/src/model/model_element.dart
+++ b/lib/src/model/model_element.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 /// The models used to represent Dart code.
 library dartdoc.models;
 
diff --git a/lib/src/model/model_function.dart b/lib/src/model/model_function.dart
index 236f1e6ea6..419f2fd9e1 100644
--- a/lib/src/model/model_function.dart
+++ b/lib/src/model/model_function.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:dartdoc/src/element_type.dart';
 import 'package:dartdoc/src/model/comment_referable.dart';
diff --git a/lib/src/model/model_node.dart b/lib/src/model/model_node.dart
index af9e9882eb..dbd48274b1 100644
--- a/lib/src/model/model_node.dart
+++ b/lib/src/model/model_node.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/file_system/file_system.dart';
diff --git a/lib/src/model/nameable.dart b/lib/src/model/nameable.dart
index 761ea0150c..8843244534 100644
--- a/lib/src/model/nameable.dart
+++ b/lib/src/model/nameable.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:collection/collection.dart';
 
 import 'locatable.dart';
diff --git a/lib/src/model/never.dart b/lib/src/model/never.dart
index f3099925e2..ad67fa774e 100644
--- a/lib/src/model/never.dart
+++ b/lib/src/model/never.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:dartdoc/src/model/comment_referable.dart';
 import 'package:dartdoc/src/model/model.dart';
diff --git a/lib/src/model/operator.dart b/lib/src/model/operator.dart
index 68551e541c..77b58e384f 100644
--- a/lib/src/model/operator.dart
+++ b/lib/src/model/operator.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/member.dart' show Member;
 import 'package:dartdoc/src/comment_references/parser.dart';
diff --git a/lib/src/model/package.dart b/lib/src/model/package.dart
index 327207a6b8..e0bb6e4a8d 100644
--- a/lib/src/model/package.dart
+++ b/lib/src/model/package.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:dartdoc/src/comment_references/model_comment_reference.dart';
diff --git a/lib/src/model/package_builder.dart b/lib/src/model/package_builder.dart
index d2e90ff489..ffeb4dfdf1 100644
--- a/lib/src/model/package_builder.dart
+++ b/lib/src/model/package_builder.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 
 import 'package:analyzer/dart/analysis/analysis_context_collection.dart';
diff --git a/lib/src/model/package_graph.dart b/lib/src/model/package_graph.dart
index 7ac8c07122..d0ea3e7b29 100644
--- a/lib/src/model/package_graph.dart
+++ b/lib/src/model/package_graph.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart' hide CommentReference;
diff --git a/lib/src/model/parameter.dart b/lib/src/model/parameter.dart
index 0d39fa89a4..ce53e205fa 100644
--- a/lib/src/model/parameter.dart
+++ b/lib/src/model/parameter.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/member.dart' show ParameterMember;
 import 'package:dartdoc/src/element_type.dart';
diff --git a/lib/src/model/prefix.dart b/lib/src/model/prefix.dart
index 627ab555d6..0e97aeaf4a 100644
--- a/lib/src/model/prefix.dart
+++ b/lib/src/model/prefix.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/scope.dart';
 import 'package:analyzer/src/dart/element/element.dart';
diff --git a/lib/src/model/privacy.dart b/lib/src/model/privacy.dart
index 9961eb0dda..0b00685aa7 100644
--- a/lib/src/model/privacy.dart
+++ b/lib/src/model/privacy.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 /// Classes implementing this have a public/private distinction.
 abstract class Privacy {
   bool get isPublic;
diff --git a/lib/src/model/source_code_mixin.dart b/lib/src/model/source_code_mixin.dart
index 1c09f98d13..c36b5cb244 100644
--- a/lib/src/model/source_code_mixin.dart
+++ b/lib/src/model/source_code_mixin.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/source/line_info.dart';
 import 'package:dartdoc/src/model/model.dart';
diff --git a/lib/src/model/top_level_container.dart b/lib/src/model/top_level_container.dart
index 0f29d608cb..183e6f1620 100644
--- a/lib/src/model/top_level_container.dart
+++ b/lib/src/model/top_level_container.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/model/model.dart';
 import 'package:dartdoc/src/model_utils.dart' as model_utils;
 
diff --git a/lib/src/model/top_level_variable.dart b/lib/src/model/top_level_variable.dart
index 3046a60b77..ea61044977 100644
--- a/lib/src/model/top_level_variable.dart
+++ b/lib/src/model/top_level_variable.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:dartdoc/src/model/comment_referable.dart';
 import 'package:dartdoc/src/model/feature.dart';
diff --git a/lib/src/model/type_parameter.dart b/lib/src/model/type_parameter.dart
index 54056ed6e0..100d4d895f 100644
--- a/lib/src/model/type_parameter.dart
+++ b/lib/src/model/type_parameter.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:dartdoc/src/element_type.dart';
 import 'package:dartdoc/src/model/comment_referable.dart';
diff --git a/lib/src/model/typedef.dart b/lib/src/model/typedef.dart
index 7c47548a50..3398db83e4 100644
--- a/lib/src/model/typedef.dart
+++ b/lib/src/model/typedef.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:dartdoc/src/element_type.dart';
diff --git a/lib/src/model_utils.dart b/lib/src/model_utils.dart
index 7a94cb7848..2f8b408a7d 100644
--- a/lib/src/model_utils.dart
+++ b/lib/src/model_utils.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.model_utils;
 
 import 'dart:convert';
diff --git a/lib/src/mustachio/annotations.dart b/lib/src/mustachio/annotations.dart
index 17e837983e..e7854432ec 100644
--- a/lib/src/mustachio/annotations.dart
+++ b/lib/src/mustachio/annotations.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 // See the Mustachio README at tool/mustachio/README.md for high-level
 // documentation.
 
diff --git a/lib/src/mustachio/parser.dart b/lib/src/mustachio/parser.dart
index 296f9861e1..a33e3c10f7 100644
--- a/lib/src/mustachio/parser.dart
+++ b/lib/src/mustachio/parser.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 // See the Mustachio README at tool/mustachio/README.md for high-level
 // documentation.
 
diff --git a/lib/src/mustachio/renderer_base.dart b/lib/src/mustachio/renderer_base.dart
index 85fcfa8f9d..072594a264 100644
--- a/lib/src/mustachio/renderer_base.dart
+++ b/lib/src/mustachio/renderer_base.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 // See the Mustachio README at tool/mustachio/README.md for high-level
 // documentation.
 
diff --git a/lib/src/package_config_provider.dart b/lib/src/package_config_provider.dart
index f82922ce0c..61a0a50116 100644
--- a/lib/src/package_config_provider.dart
+++ b/lib/src/package_config_provider.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:io' as io;
 
 import 'package:analyzer/file_system/file_system.dart';
diff --git a/lib/src/package_meta.dart b/lib/src/package_meta.dart
index 439e9c6d8c..d481f6019f 100644
--- a/lib/src/package_meta.dart
+++ b/lib/src/package_meta.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.package_meta;
 
 import 'dart:convert';
diff --git a/lib/src/quiver.dart b/lib/src/quiver.dart
index dd36843303..4e79c2883d 100644
--- a/lib/src/quiver.dart
+++ b/lib/src/quiver.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 /// Methods in-lined from package:quiver.
 
 // From lib/iterables.dart:
diff --git a/lib/src/render/category_renderer.dart b/lib/src/render/category_renderer.dart
index 0b6e838bc4..45d6db7eec 100644
--- a/lib/src/render/category_renderer.dart
+++ b/lib/src/render/category_renderer.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/model/category.dart';
 
 /// A renderer for a [Category].
diff --git a/lib/src/render/documentation_renderer.dart b/lib/src/render/documentation_renderer.dart
index 47b2351238..aab5206137 100644
--- a/lib/src/render/documentation_renderer.dart
+++ b/lib/src/render/documentation_renderer.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:html/parser.dart' show parse;
 import 'package:markdown/markdown.dart' as md;
 import 'package:meta/meta.dart';
diff --git a/lib/src/render/element_type_renderer.dart b/lib/src/render/element_type_renderer.dart
index 5ef50c24af..8e0aed669e 100644
--- a/lib/src/render/element_type_renderer.dart
+++ b/lib/src/render/element_type_renderer.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/dartdoc.dart';
 import 'package:dartdoc/src/render/parameter_renderer.dart';
 
diff --git a/lib/src/render/enum_field_renderer.dart b/lib/src/render/enum_field_renderer.dart
index 99f370fbd5..b8f847a9c4 100644
--- a/lib/src/render/enum_field_renderer.dart
+++ b/lib/src/render/enum_field_renderer.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/model/enum.dart';
 
 abstract class EnumFieldRenderer {
diff --git a/lib/src/render/feature_renderer.dart b/lib/src/render/feature_renderer.dart
index 7cce8145ac..3eb18c1b60 100644
--- a/lib/src/render/feature_renderer.dart
+++ b/lib/src/render/feature_renderer.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:convert';
 
 import 'package:dartdoc/src/model/annotation.dart';
diff --git a/lib/src/render/language_feature_renderer.dart b/lib/src/render/language_feature_renderer.dart
index 5117d71623..24972ab9d9 100644
--- a/lib/src/render/language_feature_renderer.dart
+++ b/lib/src/render/language_feature_renderer.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/model/language_feature.dart';
 
 /// A renderer for a [LanguageFeature].
diff --git a/lib/src/render/model_element_renderer.dart b/lib/src/render/model_element_renderer.dart
index 63885667ef..b37bc20de9 100644
--- a/lib/src/render/model_element_renderer.dart
+++ b/lib/src/render/model_element_renderer.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/model/feature.dart';
 import 'package:dartdoc/src/model/model_element.dart';
 
diff --git a/lib/src/render/parameter_renderer.dart b/lib/src/render/parameter_renderer.dart
index b1e551e758..df0f17a970 100644
--- a/lib/src/render/parameter_renderer.dart
+++ b/lib/src/render/parameter_renderer.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:convert';
 
 import 'package:analyzer/dart/element/type.dart';
diff --git a/lib/src/render/renderer_factory.dart b/lib/src/render/renderer_factory.dart
index 9bc6d3ba11..a03f1bbfa4 100644
--- a/lib/src/render/renderer_factory.dart
+++ b/lib/src/render/renderer_factory.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/dartdoc.dart';
 import 'package:dartdoc/src/render/category_renderer.dart';
 import 'package:dartdoc/src/render/documentation_renderer.dart';
diff --git a/lib/src/render/source_code_renderer.dart b/lib/src/render/source_code_renderer.dart
index 0a15830d78..91ffec2d73 100644
--- a/lib/src/render/source_code_renderer.dart
+++ b/lib/src/render/source_code_renderer.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:convert';
 
 /// Renderer for source code snippets extracted from source files.
diff --git a/lib/src/render/template_renderer.dart b/lib/src/render/template_renderer.dart
index 24da3e01ca..73475e2a70 100644
--- a/lib/src/render/template_renderer.dart
+++ b/lib/src/render/template_renderer.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 abstract class LayoutRenderer {
   String composeLayoutTitle(String name, String kind, bool isDeprecated);
 }
diff --git a/lib/src/render/type_parameters_renderer.dart b/lib/src/render/type_parameters_renderer.dart
index 26d947a103..c205f79f2b 100644
--- a/lib/src/render/type_parameters_renderer.dart
+++ b/lib/src/render/type_parameters_renderer.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/model/type_parameter.dart';
 
 abstract class TypeParametersRenderer {
diff --git a/lib/src/render/typedef_renderer.dart b/lib/src/render/typedef_renderer.dart
index 0a0e224716..a491744c7b 100644
--- a/lib/src/render/typedef_renderer.dart
+++ b/lib/src/render/typedef_renderer.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/model/type_parameter.dart';
 import 'package:dartdoc/src/model/typedef.dart';
 
diff --git a/lib/src/source_linker.dart b/lib/src/source_linker.dart
index 26d07329c5..5fc2313ce1 100644
--- a/lib/src/source_linker.dart
+++ b/lib/src/source_linker.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 /// A library for getting external source code links for Dartdoc.
 library dartdoc.source_linker;
 
diff --git a/lib/src/special_elements.dart b/lib/src/special_elements.dart
index 67b45197b4..a58297ca4b 100644
--- a/lib/src/special_elements.dart
+++ b/lib/src/special_elements.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 /// Handling for special elements within Dart.  When identified these
 /// may alter the interpretation and documentation generated for other
 /// [ModelElement]s.
diff --git a/lib/src/tool_definition.dart b/lib/src/tool_definition.dart
index 039d501e5d..95046d815e 100644
--- a/lib/src/tool_definition.dart
+++ b/lib/src/tool_definition.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 
 import 'package:analyzer/file_system/file_system.dart';
diff --git a/lib/src/tool_runner.dart b/lib/src/tool_runner.dart
index 1cfa337f41..bc24e3fad8 100644
--- a/lib/src/tool_runner.dart
+++ b/lib/src/tool_runner.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.tool_runner;
 
 import 'dart:io' show Process, ProcessException;
diff --git a/lib/src/tuple.dart b/lib/src/tuple.dart
index e4e1dfe926..a0655e03b8 100644
--- a/lib/src/tuple.dart
+++ b/lib/src/tuple.dart
@@ -1,5 +1,7 @@
 // Copied from source at github.com/kseo/tuple/blob/470ed3aeb/lib/src/tuple.dart
 
+// @dart=2.9
+
 // Original copyright:
 // Copyright (c) 2014, the tuple project authors. Please see the AUTHORS
 // file for details. All rights reserved. Use of this source code is governed
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index 0975566d07..5819b6d3c3 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -1,6 +1,9 @@
 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
+
+// @dart=2.9
+
 library dartdoc.utils;
 
 final RegExp leadingWhiteSpace = RegExp(r'^([ \t]*)[^ ]');
diff --git a/lib/src/warnings.dart b/lib/src/warnings.dart
index 505b8d5bea..f34b790e72 100644
--- a/lib/src/warnings.dart
+++ b/lib/src/warnings.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:collection';
 
 import 'package:analyzer/dart/element/element.dart';
diff --git a/pubspec.yaml b/pubspec.yaml
index d9d6e1e6b6..f421247c86 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -4,13 +4,13 @@ version: 1.0.2
 description: A non-interactive HTML documentation generator for Dart source code.
 homepage: https://github.com/dart-lang/dartdoc
 environment:
-  sdk: '>=2.11.99 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   analyzer: ^2.0.0
   args: ^2.0.0
   charcode: ^1.2.0
-  collection: ^1.2.0
+  collection: ^1.15.0-nullsafety.4
   cli_util: ^0.3.0
   crypto: ^3.0.0
   glob: ^2.0.0
diff --git a/test/comment_referable/comment_referable_test.dart b/test/comment_referable/comment_referable_test.dart
index 127d6262ef..84843b542c 100644
--- a/test/comment_referable/comment_referable_test.dart
+++ b/test/comment_referable/comment_referable_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.comment_reference_test;
 
 import 'package:analyzer/dart/element/element.dart';
diff --git a/test/comment_referable/parser_test.dart b/test/comment_referable/parser_test.dart
index b716408be7..7f35c82ea3 100644
--- a/test/comment_referable/parser_test.dart
+++ b/test/comment_referable/parser_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/comment_references/parser.dart';
 import 'package:test/test.dart';
 
diff --git a/test/dartdoc_options_test.dart b/test/dartdoc_options_test.dart
index f10f85810a..95f7d38bab 100644
--- a/test/dartdoc_options_test.dart
+++ b/test/dartdoc_options_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.options_test;
 
 import 'package:analyzer/file_system/file_system.dart';
diff --git a/test/documentation_comment_test.dart b/test/documentation_comment_test.dart
index d6b5deb679..1650f759d9 100644
--- a/test/documentation_comment_test.dart
+++ b/test/documentation_comment_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/src/test_utilities/mock_sdk.dart';
diff --git a/test/end2end/dartdoc_integration_test.dart b/test/end2end/dartdoc_integration_test.dart
index c8bddf94ab..c26398e314 100644
--- a/test/end2end/dartdoc_integration_test.dart
+++ b/test/end2end/dartdoc_integration_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.dartdoc_integration_test;
 
 import 'dart:async';
diff --git a/test/end2end/dartdoc_test.dart b/test/end2end/dartdoc_test.dart
index 91ccfc83f1..58640eee44 100644
--- a/test/end2end/dartdoc_test.dart
+++ b/test/end2end/dartdoc_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.dartdoc_test;
 
 import 'dart:async';
diff --git a/test/end2end/model_special_cases_test.dart b/test/end2end/model_special_cases_test.dart
index 159decd1a9..2ced723a72 100644
--- a/test/end2end/model_special_cases_test.dart
+++ b/test/end2end/model_special_cases_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 /// This test library handles checks against the model for configurations
 /// that require different PackageGraph configurations.  Since those
 /// take a long time to initialize, isolate them here to keep model_test
diff --git a/test/end2end/model_test.dart b/test/end2end/model_test.dart
index 9959891cc5..769ecd2935 100644
--- a/test/end2end/model_test.dart
+++ b/test/end2end/model_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.model_test;
 
 import 'dart:io';
diff --git a/test/grind_test.dart b/test/grind_test.dart
index 23087548cb..154cc7f3f5 100644
--- a/test/grind_test.dart
+++ b/test/grind_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.io_utils_test;
 
 import 'package:test/test.dart';
diff --git a/test/html_generator_test.dart b/test/html_generator_test.dart
index 899fb1b49d..291b1785a1 100644
--- a/test/html_generator_test.dart
+++ b/test/html_generator_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:dartdoc/dartdoc.dart';
diff --git a/test/io_utils_test.dart b/test/io_utils_test.dart
index ee24db4243..119d0421b8 100644
--- a/test/io_utils_test.dart
+++ b/test/io_utils_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.io_utils_test;
 
 import 'package:dartdoc/src/io_utils.dart';
diff --git a/test/library_test.dart b/test/library_test.dart
index fc25c8e7a3..4dff2f197a 100644
--- a/test/library_test.dart
+++ b/test/library_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:dartdoc/src/model/model.dart';
 import 'package:dartdoc/src/package_config_provider.dart';
diff --git a/test/markdown_processor_test.dart b/test/markdown_processor_test.dart
index 61a900bad2..9a5dce0a83 100644
--- a/test/markdown_processor_test.dart
+++ b/test/markdown_processor_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.markdown_processor_test;
 
 import 'package:dartdoc/src/markdown_processor.dart';
diff --git a/test/model_utils_test.dart b/test/model_utils_test.dart
index 850c2d4901..1705a51362 100644
--- a/test/model_utils_test.dart
+++ b/test/model_utils_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.model_utils_test;
 
 import 'package:dartdoc/src/model_utils.dart';
diff --git a/test/mustachio/aot_compiler_builder_test.dart b/test/mustachio/aot_compiler_builder_test.dart
index aa80b5cc66..33968513ac 100644
--- a/test/mustachio/aot_compiler_builder_test.dart
+++ b/test/mustachio/aot_compiler_builder_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 @Timeout.factor(4)
 import 'dart:convert';
 import 'package:analyzer/dart/element/element.dart';
diff --git a/test/mustachio/aot_compiler_render_test.dart b/test/mustachio/aot_compiler_render_test.dart
index 1fafe717eb..37469af23b 100644
--- a/test/mustachio/aot_compiler_render_test.dart
+++ b/test/mustachio/aot_compiler_render_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 import 'dart:convert' show json, utf8;
 import 'dart:io';
diff --git a/test/mustachio/builder_test_base.dart b/test/mustachio/builder_test_base.dart
index 59a7ea9206..5a0fce3d9e 100644
--- a/test/mustachio/builder_test_base.dart
+++ b/test/mustachio/builder_test_base.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:build/build.dart';
 import 'package:build_test/build_test.dart';
diff --git a/test/mustachio/foo.aot_renderers_for_html.dart b/test/mustachio/foo.aot_renderers_for_html.dart
index d75402395d..c56403bfdb 100644
--- a/test/mustachio/foo.aot_renderers_for_html.dart
+++ b/test/mustachio/foo.aot_renderers_for_html.dart
@@ -7,6 +7,7 @@
 // the variable is not used; generally when the section is checking if a
 // non-bool, non-Iterable field is non-null.
 // ignore_for_file: unused_local_variable
+// @dart=2.9
 
 import 'dart:convert' as _i2;
 
diff --git a/test/mustachio/foo.aot_renderers_for_md.dart b/test/mustachio/foo.aot_renderers_for_md.dart
index e2e7c00b0a..ff78fe8ba1 100644
--- a/test/mustachio/foo.aot_renderers_for_md.dart
+++ b/test/mustachio/foo.aot_renderers_for_md.dart
@@ -7,6 +7,7 @@
 // the variable is not used; generally when the section is checking if a
 // non-bool, non-Iterable field is non-null.
 // ignore_for_file: unused_local_variable
+// @dart=2.9
 
 import 'dart:convert' as _i2;
 
diff --git a/test/mustachio/foo.dart b/test/mustachio/foo.dart
index 08d046155d..3ef2a46f2f 100644
--- a/test/mustachio/foo.dart
+++ b/test/mustachio/foo.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 @Renderer.forTest(#renderFoo, Context<Foo>(), 'foo',
     visibleTypes: {Property1, Property2, Property3})
 @Renderer.forTest(#renderBar, Context<Bar>(), 'bar')
diff --git a/test/mustachio/foo.runtime_renderers.dart b/test/mustachio/foo.runtime_renderers.dart
index 118a2c88b0..6a4960555b 100644
--- a/test/mustachio/foo.runtime_renderers.dart
+++ b/test/mustachio/foo.runtime_renderers.dart
@@ -5,6 +5,7 @@
 
 // ignore_for_file: camel_case_types, deprecated_member_use_from_same_package
 // ignore_for_file: unused_import
+// @dart=2.9
 import 'package:dartdoc/dartdoc.dart';
 import 'package:dartdoc/src/generator/template_data.dart';
 import 'package:dartdoc/src/model/annotation.dart';
diff --git a/test/mustachio/parser_test.dart b/test/mustachio/parser_test.dart
index a601a1d8f4..b612db6b25 100644
--- a/test/mustachio/parser_test.dart
+++ b/test/mustachio/parser_test.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:dartdoc/src/mustachio/parser.dart';
 import 'package:test/test.dart';
 
diff --git a/test/mustachio/render_tests_test.dart b/test/mustachio/render_tests_test.dart
index 1282dd43fc..2b409addfa 100644
--- a/test/mustachio/render_tests_test.dart
+++ b/test/mustachio/render_tests_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:io';
 import 'dart:isolate' show Isolate;
 
diff --git a/test/mustachio/renderers_output_test.dart b/test/mustachio/renderers_output_test.dart
index b1c3ec4446..b26c462750 100644
--- a/test/mustachio/renderers_output_test.dart
+++ b/test/mustachio/renderers_output_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.dartdoc_test;
 
 import 'dart:async';
diff --git a/test/mustachio/runtime_renderer_builder_test.dart b/test/mustachio/runtime_renderer_builder_test.dart
index 65dcf65c96..d84cb543fb 100644
--- a/test/mustachio/runtime_renderer_builder_test.dart
+++ b/test/mustachio/runtime_renderer_builder_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 @Timeout.factor(2)
 import 'dart:convert';
 import 'package:analyzer/dart/element/element.dart';
diff --git a/test/mustachio/runtime_renderer_render_test.dart b/test/mustachio/runtime_renderer_render_test.dart
index a0dd698376..456423b9a3 100644
--- a/test/mustachio/runtime_renderer_render_test.dart
+++ b/test/mustachio/runtime_renderer_render_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:dartdoc/src/mustachio/renderer_base.dart';
diff --git a/test/package_meta_test.dart b/test/package_meta_test.dart
index 7339818a90..3c1c262097 100644
--- a/test/package_meta_test.dart
+++ b/test/package_meta_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.package_utils_test;
 
 import 'package:dartdoc/src/io_utils.dart';
diff --git a/test/package_test.dart b/test/package_test.dart
index ca93d97fae..4c44da5178 100644
--- a/test/package_test.dart
+++ b/test/package_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:dartdoc/src/dartdoc_options.dart';
diff --git a/test/quiver_test.dart b/test/quiver_test.dart
index eb8c2a98e7..341cce4568 100644
--- a/test/quiver_test.dart
+++ b/test/quiver_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/quiver.dart';
 import 'package:test/test.dart';
 
diff --git a/test/render/template_renderer_test.dart b/test/render/template_renderer_test.dart
index 379da3e0aa..4d43a1daf5 100644
--- a/test/render/template_renderer_test.dart
+++ b/test/render/template_renderer_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:dartdoc/src/render/template_renderer.dart';
 import 'package:test/test.dart';
 
diff --git a/test/resource_loader_test.dart b/test/resource_loader_test.dart
index aa915ba123..2962f71ba9 100644
--- a/test/resource_loader_test.dart
+++ b/test/resource_loader_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.resource_loader_test;
 
 import 'package:analyzer/file_system/file_system.dart';
diff --git a/test/source_linker_test.dart b/test/source_linker_test.dart
index 7c68a8d96b..adc591e34a 100644
--- a/test/source_linker_test.dart
+++ b/test/source_linker_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.source_linker_test;
 
 import 'package:dartdoc/src/dartdoc_options.dart';
diff --git a/test/src/utils.dart b/test/src/utils.dart
index 779fc77839..aa89a5b491 100644
--- a/test/src/utils.dart
+++ b/test/src/utils.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library test_utils;
 
 import 'dart:io';
diff --git a/test/tool_runner_test.dart b/test/tool_runner_test.dart
index 71515b1c70..015b50f0d1 100644
--- a/test/tool_runner_test.dart
+++ b/test/tool_runner_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.model_test;
 
 import 'dart:io';
diff --git a/test/utils_test.dart b/test/utils_test.dart
index 9556f3cc28..4dcf171d2b 100644
--- a/test/utils_test.dart
+++ b/test/utils_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 library dartdoc.utils_test;
 
 import 'package:dartdoc/src/utils.dart';
diff --git a/test/warnings_test.dart b/test/warnings_test.dart
index b4db31ee41..6f63e6b497 100644
--- a/test/warnings_test.dart
+++ b/test/warnings_test.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 /// Unit tests for lib/src/warnings.dart.
 library dartdoc.warnings_test;
 
diff --git a/tool/builder.dart b/tool/builder.dart
index d77d109653..6a53d9d019 100644
--- a/tool/builder.dart
+++ b/tool/builder.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 
 import 'package:build/build.dart';
diff --git a/tool/doc_packages.dart b/tool/doc_packages.dart
index 0988498faa..b8ea616150 100644
--- a/tool/doc_packages.dart
+++ b/tool/doc_packages.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 /// A CLI tool to generate documentation for packages from pub.dartlang.org.
 library dartdoc.doc_packages;
 
diff --git a/tool/grind.dart b/tool/grind.dart
index db4701cbcc..5485e95383 100644
--- a/tool/grind.dart
+++ b/tool/grind.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 import 'dart:io' hide ProcessException;
 
@@ -316,6 +318,10 @@ void presubmit() => null;
 @Depends(presubmit, longTest, testDartdoc)
 void buildbot() => null;
 
+@Task('Run buildbot tests, but without publish test')
+@Depends(analyze, dartfmt, checkBuild, smokeTest, longTest, testDartdoc)
+void buildbotNoPublish() => null;
+
 @Task('Generate docs for the Dart SDK')
 Future buildSdkDocs() async {
   log('building SDK docs');
diff --git a/tool/mustachio/builder.dart b/tool/mustachio/builder.dart
index 85ed810b8e..17a39abae1 100644
--- a/tool/mustachio/builder.dart
+++ b/tool/mustachio/builder.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/member.dart';
diff --git a/tool/mustachio/codegen_aot_compiler.dart b/tool/mustachio/codegen_aot_compiler.dart
index 2bd4b84a09..1e12843d15 100644
--- a/tool/mustachio/codegen_aot_compiler.dart
+++ b/tool/mustachio/codegen_aot_compiler.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/dart/element/type_provider.dart';
@@ -67,6 +69,7 @@ Future<String> compileTemplatesToRenderers(
 // the variable is not used; generally when the section is checking if a
 // non-bool, non-Iterable field is non-null.
 // ignore_for_file: unused_local_variable
+// @dart=2.9
 
 ${library.accept(DartEmitter.scoped(orderDirectives: true))}
 ''');
diff --git a/tool/mustachio/codegen_runtime_renderer.dart b/tool/mustachio/codegen_runtime_renderer.dart
index 4636e2478c..162c6a99b6 100644
--- a/tool/mustachio/codegen_runtime_renderer.dart
+++ b/tool/mustachio/codegen_runtime_renderer.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:collection';
 
 import 'package:analyzer/dart/element/element.dart';
@@ -81,6 +83,7 @@ class RuntimeRenderersBuilder {
 
 // ignore_for_file: camel_case_types, deprecated_member_use_from_same_package
 // ignore_for_file: unused_import
+// @dart=2.9
 import 'package:dartdoc/dartdoc.dart';
 import 'package:dartdoc/src/generator/template_data.dart';
 import 'package:dartdoc/src/model/annotation.dart';
diff --git a/tool/mustachio/utilities.dart b/tool/mustachio/utilities.dart
index 983c421a5e..920a6592b3 100644
--- a/tool/mustachio/utilities.dart
+++ b/tool/mustachio/utilities.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:analyzer/dart/element/element.dart';
 
 extension ClassExtensions on ClassElement {
diff --git a/tool/subprocess_launcher.dart b/tool/subprocess_launcher.dart
index e25646e753..4fc793fab4 100644
--- a/tool/subprocess_launcher.dart
+++ b/tool/subprocess_launcher.dart
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
diff --git a/tool/travis.sh b/tool/travis.sh
index 17f5607930..f9c33eb78e 100755
--- a/tool/travis.sh
+++ b/tool/travis.sh
@@ -42,6 +42,9 @@ elif [ "$DARTDOC_BOT" = "sdk-analyzer" ]; then
   echo "Running all tests against the SDK analyzer"
   unset COVERAGE_TOKEN
   pub run grinder test-with-analyzer-sdk
+elif [ "$DARTDOC_BOT" = "nnbd" ]; then
+  echo "Running main dartdoc bot excluding publish test"
+  pub run grinder buildbot-no-publish
 else
   echo "Running main dartdoc bot"
   pub run grinder buildbot