diff --git a/tool/builder.dart b/tool/builder.dart
index 31e7804ee5..c9d1aad05a 100644
--- a/tool/builder.dart
+++ b/tool/builder.dart
@@ -2,8 +2,6 @@
 // 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 b8ea616150..244798f4a1 100644
--- a/tool/doc_packages.dart
+++ b/tool/doc_packages.dart
@@ -2,8 +2,6 @@
 // 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;
 
@@ -86,7 +84,7 @@ void performGenerate(int page) {
 
   _packageUrls(page).then((List<String> packages) {
     return _getPackageInfos(packages).then((List<PackageInfo> infos) {
-      return Future.forEach(infos, (info) {
+      return Future.forEach(infos, (PackageInfo info) {
         return _printGenerationResult(info, _generateFor(info));
       });
     });
@@ -147,7 +145,7 @@ Future<List<PackageInfo>> _getPackageInfos(List<String> packageUrls) {
   return Future.wait(futures);
 }
 
-StringBuffer _logBuffer;
+StringBuffer? _logBuffer;
 
 /// Generate the docs for the given package into _rootDir. Return whether
 /// generation was performed or was skipped (due to an older package).
@@ -195,7 +193,7 @@ Future<bool> _generateFor(PackageInfo package) async {
 }
 
 Future<void> _exec(String command, List<String> args,
-    {String cwd,
+    {String? cwd,
     bool quiet = false,
     Duration timeout = const Duration(seconds: 60)}) {
   return Process.start(command, args, workingDirectory: cwd)
@@ -209,20 +207,16 @@ Future<void> _exec(String command, List<String> args,
       if (code != 0) throw code;
     });
 
-    if (timeout != null) {
-      return f.timeout(timeout, onTimeout: () {
-        _log('Timing out operation $command.');
-        process.kill();
-        throw 'timeout on $command';
-      });
-    } else {
-      return f;
-    }
+    return f.timeout(timeout, onTimeout: () {
+      _log('Timing out operation $command.');
+      process.kill();
+      throw 'timeout on $command';
+    });
   });
 }
 
 bool _isOldSdkConstraint(Map<String, dynamic> pubspecInfo) {
-  var environment = pubspecInfo['environment'] as Map;
+  var environment = pubspecInfo['environment'] as Map?;
   if (environment != null) {
     var sdk = environment['sdk'];
     if (sdk != null) {
@@ -243,8 +237,10 @@ bool _isOldSdkConstraint(Map<String, dynamic> pubspecInfo) {
   return false;
 }
 
+/// Log entries will be dropped if [_logBuffer] has not been initialized.
 void _log(String str) {
-  _logBuffer.write(str);
+  assert(_logBuffer != null);
+  _logBuffer?.write(str);
 }
 
 class PackageInfo {
diff --git a/tool/grind.dart b/tool/grind.dart
index a1ebc02b6b..c8653315b9 100644
--- a/tool/grind.dart
+++ b/tool/grind.dart
@@ -553,7 +553,7 @@ Future<void> testWithAnalyzerSdk() async {
       workingDirectory: sdkDartdoc);
 }
 
-Future<List<Map<Object, Object>>> _buildSdkDocs(
+Future<Iterable<Map<String, Object>>> _buildSdkDocs(
     String sdkDocsPath, Future<String> futureCwd,
     [String label]) async {
   label ??= '';
@@ -576,7 +576,7 @@ Future<List<Map<Object, Object>>> _buildSdkDocs(
       workingDirectory: cwd);
 }
 
-Future<List<Map<Object, Object>>> _buildTestPackageDocs(
+Future<Iterable<Map<String, Object>>> _buildTestPackageDocs(
     String outputDir, String cwd,
     {List<String> params, String label = '', String testPackagePath}) async {
   if (label != '') label = '-$label';
@@ -924,7 +924,7 @@ class FlutterRepo {
   SubprocessLauncher launcher;
 }
 
-Future<List<Map<Object, Object>>> _buildFlutterDocs(
+Future<Iterable<Map<String, Object>>> _buildFlutterDocs(
     String flutterPath, Future<String> futureCwd, Map<String, String> env,
     [String label]) async {
   var flutterRepo = await FlutterRepo.copyFromExistingFlutterRepo(
diff --git a/tool/subprocess_launcher.dart b/tool/subprocess_launcher.dart
index 679427ad70..c0ada7434d 100644
--- a/tool/subprocess_launcher.dart
+++ b/tool/subprocess_launcher.dart
@@ -2,8 +2,6 @@
 // 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';
@@ -14,11 +12,9 @@ import 'package:path/path.dart' as p;
 /// Keeps track of coverage data automatically for any processes run by this
 /// [CoverageSubprocessLauncher].  Requires that these be dart processes.
 class CoverageSubprocessLauncher extends SubprocessLauncher {
-  CoverageSubprocessLauncher(String context, [Map<String, String> environment])
-      : super(context, environment) {
-    environment ??= {};
-    environment['DARTDOC_COVERAGE_DATA'] = tempDir.path;
-  }
+  CoverageSubprocessLauncher(String context, [Map<String, String>? environment])
+      : super(
+            context, {...?environment, 'DARTDOC_COVERAGE_DATA': tempDir.path});
 
   static int nextRun = 0;
 
@@ -31,24 +27,20 @@ class CoverageSubprocessLauncher extends SubprocessLauncher {
   // is enabled.
   static List<Future<Iterable<Map<Object, Object>>>> coverageResults = [];
 
-  static Directory _tempDir;
-  static Directory get tempDir {
-    if (_tempDir == null) {
-      if (Platform.environment['DARTDOC_COVERAGE_DATA'] != null) {
-        _tempDir = Directory(Platform.environment['DARTDOC_COVERAGE_DATA']);
-      } else {
-        _tempDir = Directory.systemTemp.createTempSync('dartdoc_coverage_data');
-      }
+  static Directory tempDir = () {
+    var coverageData = Platform.environment['DARTDOC_COVERAGE_DATA'];
+    if (coverageData != null) {
+      return Directory(coverageData);
     }
-    return _tempDir;
-  }
+    return Directory.systemTemp.createTempSync('dartdoc_coverage_data');
+  }();
 
   static String buildNextCoverageFilename() =>
       p.join(tempDir.path, 'dart-cov-$pid-${nextRun++}.json');
 
   /// Call once all coverage runs have been generated by calling runStreamed
   /// on all [CoverageSubprocessLaunchers].
-  static Future<void> generateCoverageToFile(
+  static Future<dynamic> generateCoverageToFile(
       File outputFile, ResourceProvider resourceProvider) async {
     if (!coverageEnabled) return Future.value(null);
     var currentCoverageResults = coverageResults;
@@ -75,12 +67,12 @@ class CoverageSubprocessLauncher extends SubprocessLauncher {
   }
 
   @override
-  Future<Iterable<Map<Object, Object>>> runStreamed(
+  Future<Iterable<Map<String, Object>>> runStreamed(
       String executable, List<String> arguments,
-      {String workingDirectory,
-      Map<String, String> environment,
+      {String? workingDirectory,
+      Map<String, String>? environment,
       bool includeParentEnvironment = true,
-      void Function(String) perLine}) async {
+      void Function(String)? perLine}) async {
     environment ??= {};
     assert(
         executable == Platform.executable ||
@@ -91,16 +83,17 @@ class CoverageSubprocessLauncher extends SubprocessLauncher {
     void parsePortAsString(String line) {
       if (!portAsString.isCompleted && coverageEnabled) {
         var m = _observatoryPortRegexp.matchAsPrefix(line);
-        if (m?.group(1) != null) portAsString.complete(m.group(1));
+        if (m != null) {
+          if (m.group(1) != null) portAsString.complete(m.group(1));
+        }
       } else {
         if (perLine != null) perLine(line);
       }
     }
 
-    Completer<Iterable<Map<Object, Object>>> coverageResult;
+    Completer<Iterable<Map<Object, Object>>> coverageResult = Completer();
 
     if (coverageEnabled) {
-      coverageResult = Completer();
       // This must be added before awaiting in this method.
       coverageResults.add(coverageResult.future);
       arguments = [
@@ -139,29 +132,27 @@ class CoverageSubprocessLauncher extends SubprocessLauncher {
 
 class SubprocessLauncher {
   final String context;
-  final Map<String, String> environmentDefaults;
+  final Map<String, String> environmentDefaults = {};
 
   String get prefix => context.isNotEmpty ? '$context: ' : '';
 
   // from flutter:dev/tools/dartdoc.dart, modified
   static Future<void> _printStream(Stream<List<int>> stream, Stdout output,
-      {String prefix = '', Iterable<String> Function(String line) filter}) {
-    assert(prefix != null);
-    filter ??= (line) => [line];
+      {String prefix = '',
+      required Iterable<String> Function(String line) filter}) {
     return stream
         .transform(utf8.decoder)
         .transform(const LineSplitter())
         .expand(filter)
         .listen((String line) {
-      if (line != null) {
-        output.write('$prefix$line'.trim());
-        output.write('\n');
-      }
+      output.write('$prefix$line'.trim());
+      output.write('\n');
     }).asFuture();
   }
 
-  SubprocessLauncher(this.context, [Map<String, String> environment])
-      : environmentDefaults = environment ?? <String, String>{};
+  SubprocessLauncher(this.context, [Map<String, String>? environment]) {
+    environmentDefaults.addAll(environment ?? {});
+  }
 
   /// A wrapper around start/await process.exitCode that will display the
   /// output of the executable continuously and fail on non-zero exit codes.
@@ -173,22 +164,23 @@ class SubprocessLauncher {
   /// Windows (though some of the bashisms will no longer make sense).
   /// TODO(jcollins-g): refactor to return a stream of stderr/stdout lines
   ///                   and their associated JSON objects.
-  Future<Iterable<Map<Object, Object>>> runStreamed(
+  Future<Iterable<Map<String, Object>>> runStreamed(
       String executable, List<String> arguments,
-      {String workingDirectory,
-      Map<String, String> environment,
+      {String? workingDirectory,
+      Map<String, String>? environment,
       bool includeParentEnvironment = true,
-      void Function(String) perLine}) async {
-    environment ??= {};
-    environment.addAll(environmentDefaults);
-    List<Map<Object, Object>> jsonObjects;
+      void Function(String)? perLine}) async {
+    environment = {}
+      ..addAll(environmentDefaults)
+      ..addAll(environment ?? {});
+    List<Map<String, Object>> jsonObjects = [];
 
     /// Allow us to pretend we didn't pass the JSON flag in to dartdoc by
     /// printing what dartdoc would have printed without it, yet storing
     /// json objects into [jsonObjects].
     Iterable<String> jsonCallback(String line) {
       if (perLine != null) perLine(line);
-      Map<Object, Object> result;
+      Map<String, Object>? result;
       try {
         result = json.decoder.convert(line);
       } on FormatException {
@@ -198,10 +190,9 @@ class SubprocessLauncher {
         // line.  Just ignore it and leave result null.
       }
       if (result != null) {
-        jsonObjects ??= [];
         jsonObjects.add(result);
         if (result.containsKey('message')) {
-          line = result['message'];
+          line = result['message'] as String;
         } else if (result.containsKey('data')) {
           var data = result['data'] as Map;
           line = data['text'];
@@ -212,12 +203,12 @@ class SubprocessLauncher {
 
     stderr.write('$prefix+ ');
     if (workingDirectory != null) stderr.write('(cd "$workingDirectory" && ');
-    if (environment != null) {
-      stderr.write(environment.keys.map((String key) {
-        if (environment[key].contains(_quotables)) {
-          return "$key='${environment[key]}'";
+    if (environment.isNotEmpty) {
+      stderr.write(environment.entries.map((MapEntry<String, String> entry) {
+        if (entry.key.contains(_quotables)) {
+          return "${entry.key}='${entry.value}'";
         } else {
-          return '$key=${environment[key]}';
+          return '${entry.key}=${entry.value}';
         }
       }).join(' '));
       stderr.write(' ');
@@ -235,7 +226,7 @@ class SubprocessLauncher {
     if (workingDirectory != null) stderr.write(')');
     stderr.write('\n');
 
-    if (Platform.environment.containsKey('DRY_RUN')) return null;
+    if (Platform.environment.containsKey('DRY_RUN')) return {};
 
     var realExecutable = executable;
     var realArguments = <String>[];