diff --git a/lib/src/dartdoc_options.dart b/lib/src/dartdoc_options.dart
index f4e8da6326..c3e0d09254 100644
--- a/lib/src/dartdoc_options.dart
+++ b/lib/src/dartdoc_options.dart
@@ -242,7 +242,10 @@ class ToolConfiguration {
             setupCommand;
       }
       newToolDefinitions[name] = ToolDefinition.fromCommand(
-          [executable] + command, setupCommand, description, resourceProvider,
+          [executable] + command,
+          setupCommand ?? const [],
+          description,
+          resourceProvider,
           compileArgs: compileArgs ?? const []);
     }
     return ToolConfiguration._(newToolDefinitions, resourceProvider);
diff --git a/lib/src/tool_definition.dart b/lib/src/tool_definition.dart
index fe37380c34..44ec3116e2 100644
--- a/lib/src/tool_definition.dart
+++ b/lib/src/tool_definition.dart
@@ -23,8 +23,8 @@ class ToolDefinition {
   /// A list containing the command and options to setup phase for this tool.
   /// The first argument in the command is the tool executable, and will have
   /// its path evaluated relative to the `dartdoc_options.yaml` location. May
-  /// be null or empty, in which case it will be ignored at setup time.
-  final List<String>? setupCommand;
+  /// be empty, in which case it will be ignored at setup time.
+  final List<String> setupCommand;
 
   /// A description of the defined tool. Must not be null.
   final String description;
@@ -43,7 +43,7 @@ class ToolDefinition {
   /// given.
   factory ToolDefinition.fromCommand(
       List<String> command,
-      List<String>? setupCommand,
+      List<String> setupCommand,
       String description,
       ResourceProvider resourceProvider,
       {List<String>? compileArgs}) {
@@ -68,10 +68,9 @@ class ToolDefinition {
 
   @override
   String toString() {
-    var setupCommand = this.setupCommand;
     final commandString =
         '${this is DartToolDefinition ? '(Dart) ' : ''}"${command.join(' ')}"';
-    if (setupCommand == null) {
+    if (setupCommand.isEmpty) {
       return '$runtimeType: $commandString ($description)';
     } else {
       return '$runtimeType: $commandString, with setup command '
@@ -140,7 +139,7 @@ class DartToolDefinition extends ToolDefinition {
     }
   }
 
-  DartToolDefinition(List<String> command, List<String>? setupCommand,
+  DartToolDefinition(List<String> command, List<String> setupCommand,
       String description, this._resourceProvider,
       {this.compileArgs = const []})
       : super(command, setupCommand, description);
diff --git a/lib/src/tool_runner.dart b/lib/src/tool_runner.dart
index 47010b2813..1b214f12be 100644
--- a/lib/src/tool_runner.dart
+++ b/lib/src/tool_runner.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
-
 library dartdoc.tool_runner;
 
 import 'dart:io' show Process, ProcessException;
@@ -12,7 +10,6 @@ import 'package:analyzer/file_system/file_system.dart';
 import 'package:dartdoc/src/dartdoc_options.dart';
 import 'package:dartdoc/src/io_utils.dart';
 import 'package:dartdoc/src/tool_definition.dart';
-import 'package:meta/meta.dart';
 import 'package:path/path.dart' as p;
 
 typedef ToolErrorCallback = void Function(String message);
@@ -97,7 +94,7 @@ class ToolRunner {
   /// calls [toolErrorCallback] with a detailed error message, and returns `''`.
   Future<String> _runProcess(String name, String content, String commandPath,
       List<String> args, Map<String, String> environment,
-      {@required ToolErrorCallback toolErrorCallback}) async {
+      {required ToolErrorCallback toolErrorCallback}) async {
     String commandString() => ([commandPath] + args).join(' ');
     try {
       var result =
@@ -128,10 +125,9 @@ class ToolRunner {
   /// be sent to to the tool is given in the optional [content]. The stdout of
   /// the tool is returned.
   Future<String> run(List<String> args,
-      {@required String content,
-      @required ToolErrorCallback toolErrorCallback,
-      Map<String, String> environment}) async {
-    assert(args != null);
+      {required String content,
+      required ToolErrorCallback toolErrorCallback,
+      Map<String, String> environment = const {}}) async {
     assert(args.isNotEmpty);
     return _toolTracker.add(() {
       return _run(args,
@@ -142,13 +138,10 @@ class ToolRunner {
   }
 
   Future<String> _run(List<String> args,
-      {@required ToolErrorCallback toolErrorCallback,
-      String content,
-      Map<String, String> environment}) async {
-    assert(args != null);
+      {required ToolErrorCallback toolErrorCallback,
+      String content = '',
+      required Map<String, String> environment}) async {
     assert(args.isNotEmpty);
-    content ??= '';
-    environment ??= <String, String>{};
     var toolName = args.removeAt(0);
     if (!toolConfiguration.tools.containsKey(toolName)) {
       toolErrorCallback(
@@ -157,14 +150,14 @@ class ToolRunner {
       return '';
     }
     var toolDefinition = toolConfiguration.tools[toolName];
-    var toolArgs = toolDefinition.command;
+    var toolArgs = toolDefinition!.command;
 
     // Substitute the temp filename for the "$INPUT" token, and all of the other
     // environment variables. Variables are allowed to either be in $(VAR) form,
     // or $VAR form.
     var envWithInput = {
       'INPUT': _tmpFileWithContent(content),
-      'TOOL_COMMAND': toolDefinition.command[0],
+      'TOOL_COMMAND': toolArgs[0],
       ...environment,
     };
     if (toolDefinition is DartToolDefinition) {
@@ -176,7 +169,7 @@ class ToolRunner {
       // filesystem.
       envWithInput['DART_SNAPSHOT_CACHE'] = pathContext.absolute(
           SnapshotCache.instanceFor(resourceProvider).snapshotCache.path);
-      if (toolDefinition.setupCommand != null) {
+      if (toolDefinition.setupCommand.isNotEmpty) {
         envWithInput['DART_SETUP_COMMAND'] = toolDefinition.setupCommand[0];
       }
     }
@@ -186,7 +179,8 @@ class ToolRunner {
       ..._substituteInArgs(args, envWithInput),
     ];
 
-    if (toolDefinition.setupCommand != null && !toolDefinition.setupComplete) {
+    if (toolDefinition.setupCommand.isNotEmpty &&
+        !toolDefinition.setupComplete) {
       await _runSetup(
           toolName, toolDefinition, envWithInput, toolErrorCallback);
     }
diff --git a/test/tool_runner_test.dart b/test/tool_runner_test.dart
index afacb3deba..20985b7d99 100644
--- a/test/tool_runner_test.dart
+++ b/test/tool_runner_test.dart
@@ -88,7 +88,7 @@ echo:
     // yaml map (which would fail on a missing executable), or a file is deleted
     // during execution,it might, so we test it.
     toolMap.tools.addAll({
-      'missing': ToolDefinition(['/a/missing/executable'], null, 'missing'),
+      'missing': ToolDefinition(['/a/missing/executable'], [], 'missing'),
     });
 
     runner = ToolRunner(toolMap);