diff --git a/packages/flutter_tools/lib/src/build_system/targets/deferred_components.dart b/packages/flutter_tools/lib/src/build_system/targets/deferred_components.dart
index a01ab26bab2..21208979a66 100644
--- a/packages/flutter_tools/lib/src/build_system/targets/deferred_components.dart
+++ b/packages/flutter_tools/lib/src/build_system/targets/deferred_components.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:meta/meta.dart';
 
 import '../../android/deferred_components_gen_snapshot_validator.dart';
@@ -19,8 +17,8 @@ import 'android.dart';
 class DeferredComponentsGenSnapshotValidatorTarget extends Target {
   /// Create an [AndroidAotDeferredComponentsBundle] implementation for a given [targetPlatform] and [buildMode].
   DeferredComponentsGenSnapshotValidatorTarget({
-    @required this.deferredComponentsDependencies,
-    @required this.nonDeferredComponentsDependencies,
+    required this.deferredComponentsDependencies,
+    required this.nonDeferredComponentsDependencies,
     this.title,
     this.exitOnFail = true,
   });
@@ -31,7 +29,7 @@ class DeferredComponentsGenSnapshotValidatorTarget extends Target {
 
   /// The title of the [DeferredComponentsGenSnapshotValidator] that is
   /// displayed to the developer when logging results.
-  final String title;
+  final String? title;
 
   /// Whether to exit the tool if a recommended change is found by the
   /// [DeferredComponentsGenSnapshotValidator].
@@ -72,7 +70,7 @@ class DeferredComponentsGenSnapshotValidatorTarget extends Target {
   }
 
   @visibleForTesting
-  DeferredComponentsGenSnapshotValidator validator;
+  DeferredComponentsGenSnapshotValidator? validator;
 
   @override
   Future<void> build(Environment environment) async {
@@ -92,18 +90,18 @@ class DeferredComponentsGenSnapshotValidatorTarget extends Target {
         abis: _abis
     );
 
-    validator
+    validator!
       ..checkAppAndroidManifestComponentLoadingUnitMapping(
-          FlutterProject.current().manifest.deferredComponents,
+          FlutterProject.current().manifest.deferredComponents ?? <DeferredComponent>[],
           generatedLoadingUnits,
       )
       ..checkAgainstLoadingUnitsCache(generatedLoadingUnits)
       ..writeLoadingUnitsCache(generatedLoadingUnits);
 
-    validator.handleResults();
+    validator!.handleResults();
 
     depfileService.writeToFile(
-      Depfile(validator.inputs, validator.outputs),
+      Depfile(validator!.inputs, validator!.outputs),
       environment.buildDir.childFile('flutter_$name.d'),
     );
   }
diff --git a/packages/flutter_tools/lib/src/build_system/targets/ios.dart b/packages/flutter_tools/lib/src/build_system/targets/ios.dart
index e6a8e642dfb..ae2b2014fd2 100644
--- a/packages/flutter_tools/lib/src/build_system/targets/ios.dart
+++ b/packages/flutter_tools/lib/src/build_system/targets/ios.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:meta/meta.dart';
 
 import '../../artifacts.dart';
@@ -37,45 +35,47 @@ abstract class AotAssemblyBase extends Target {
     final AOTSnapshotter snapshotter = AOTSnapshotter(
       fileSystem: environment.fileSystem,
       logger: environment.logger,
-      xcode: globals.xcode,
+      xcode: globals.xcode!,
       artifacts: environment.artifacts,
       processManager: environment.processManager,
     );
     final String buildOutputPath = environment.buildDir.path;
-    if (environment.defines[kBuildMode] == null) {
+    final String? environmentBuildMode = environment.defines[kBuildMode];
+    if (environmentBuildMode == null) {
       throw MissingDefineException(kBuildMode, 'aot_assembly');
     }
-    if (environment.defines[kTargetPlatform] == null) {
+    final String? environmentTargetPlatform = environment.defines[kTargetPlatform];
+    if (environmentTargetPlatform== null) {
       throw MissingDefineException(kTargetPlatform, 'aot_assembly');
     }
-    if (environment.defines[kSdkRoot] == null) {
+    final String? sdkRoot = environment.defines[kSdkRoot];
+    if (sdkRoot == null) {
       throw MissingDefineException(kSdkRoot, 'aot_assembly');
     }
 
     final List<String> extraGenSnapshotOptions = decodeCommaSeparated(environment.defines, kExtraGenSnapshotOptions);
     final bool bitcode = environment.defines[kBitcodeFlag] == 'true';
-    final BuildMode buildMode = getBuildModeForName(environment.defines[kBuildMode]);
-    final TargetPlatform targetPlatform = getTargetPlatformForName(environment.defines[kTargetPlatform]);
-    final String splitDebugInfo = environment.defines[kSplitDebugInfo];
+    final BuildMode buildMode = getBuildModeForName(environmentBuildMode);
+    final TargetPlatform targetPlatform = getTargetPlatformForName(environmentTargetPlatform);
+    final String? splitDebugInfo = environment.defines[kSplitDebugInfo];
     final bool dartObfuscation = environment.defines[kDartObfuscation] == 'true';
     final List<DarwinArch> darwinArchs = environment.defines[kIosArchs]
       ?.split(' ')
-      ?.map(getIOSArchForName)
-      ?.toList()
+      .map(getIOSArchForName)
+      .toList()
       ?? <DarwinArch>[DarwinArch.arm64];
     if (targetPlatform != TargetPlatform.ios) {
       throw Exception('aot_assembly is only supported for iOS applications.');
     }
 
-    final String sdkRoot = environment.defines[kSdkRoot];
-    final EnvironmentType environmentType = environmentTypeFromSdkroot(sdkRoot, environment.fileSystem);
+    final EnvironmentType? environmentType = environmentTypeFromSdkroot(sdkRoot, environment.fileSystem);
     if (environmentType == EnvironmentType.simulator) {
       throw Exception(
         'release/profile builds are only supported for physical devices. '
         'attempted to build for simulator.'
       );
     }
-    final String codeSizeDirectory = environment.defines[kCodeSizeDirectory];
+    final String? codeSizeDirectory = environment.defines[kCodeSizeDirectory];
 
     // If we're building multiple iOS archs the binaries need to be lipo'd
     // together.
@@ -220,14 +220,13 @@ class DebugUniversalFramework extends Target {
 
   @override
   Future<void> build(Environment environment) async {
-    if (environment.defines[kSdkRoot] == null) {
+    final String? sdkRoot = environment.defines[kSdkRoot];
+    if (sdkRoot == null) {
       throw MissingDefineException(kSdkRoot, name);
     }
 
     // Generate a trivial App.framework.
-    final Set<String> iosArchNames = environment.defines[kIosArchs]
-      ?.split(' ')
-      ?.toSet();
+    final Set<String>? iosArchNames = environment.defines[kIosArchs]?.split(' ').toSet();
     final File output = environment.buildDir
       .childDirectory('App.framework')
       .childFile('App');
@@ -236,6 +235,7 @@ class DebugUniversalFramework extends Target {
       output,
       environment,
       iosArchNames,
+      sdkRoot,
     );
   }
 }
@@ -272,30 +272,31 @@ abstract class UnpackIOS extends Target {
 
   @override
   Future<void> build(Environment environment) async {
-    if (environment.defines[kSdkRoot] == null) {
+    final String? sdkRoot = environment.defines[kSdkRoot];
+    if (sdkRoot == null) {
       throw MissingDefineException(kSdkRoot, name);
     }
-    if (environment.defines[kIosArchs] == null) {
+    final String? archs = environment.defines[kIosArchs];
+    if (archs == null) {
       throw MissingDefineException(kIosArchs, name);
     }
     if (environment.defines[kBitcodeFlag] == null) {
       throw MissingDefineException(kBitcodeFlag, name);
     }
-    _copyFramework(environment);
+    _copyFramework(environment, sdkRoot);
 
     final File frameworkBinary = environment.outputDir.childDirectory('Flutter.framework').childFile('Flutter');
     final String frameworkBinaryPath = frameworkBinary.path;
     if (!frameworkBinary.existsSync()) {
       throw Exception('Binary $frameworkBinaryPath does not exist, cannot thin');
     }
-    _thinFramework(environment, frameworkBinaryPath);
+    _thinFramework(environment, frameworkBinaryPath, archs);
     _bitcodeStripFramework(environment, frameworkBinaryPath);
     _signFramework(environment, frameworkBinaryPath, buildMode);
   }
 
-  void _copyFramework(Environment environment) {
-    final String sdkRoot = environment.defines[kSdkRoot];
-    final EnvironmentType environmentType = environmentTypeFromSdkroot(sdkRoot, environment.fileSystem);
+  void _copyFramework(Environment environment, String sdkRoot) {
+    final EnvironmentType? environmentType = environmentTypeFromSdkroot(sdkRoot, environment.fileSystem);
     final String basePath = environment.artifacts.getArtifactPath(
       Artifact.flutterFramework,
       platform: TargetPlatform.ios,
@@ -321,8 +322,7 @@ abstract class UnpackIOS extends Target {
   }
 
   /// Destructively thin Flutter.framework to include only the specified architectures.
-  void _thinFramework(Environment environment, String frameworkBinaryPath) {
-    final String archs = environment.defines[kIosArchs];
+  void _thinFramework(Environment environment, String frameworkBinaryPath, String archs) {
     final List<String> archList = archs.split(' ').toList();
     final ProcessResult infoResult = environment.processManager.runSync(<String>[
       'lipo',
@@ -454,10 +454,11 @@ abstract class IosAssetBundle extends Target {
 
   @override
   Future<void> build(Environment environment) async {
-    if (environment.defines[kBuildMode] == null) {
+    final String? environmentBuildMode = environment.defines[kBuildMode];
+    if (environmentBuildMode == null) {
       throw MissingDefineException(kBuildMode, name);
     }
-    final BuildMode buildMode = getBuildModeForName(environment.defines[kBuildMode]);
+    final BuildMode buildMode = getBuildModeForName(environmentBuildMode);
     final Directory frameworkDirectory = environment.outputDir.childDirectory('App.framework');
     final String frameworkBinaryPath = frameworkDirectory.childFile('App').path;
     final Directory assetDirectory = frameworkDirectory.childDirectory('flutter_assets');
@@ -574,7 +575,7 @@ class ReleaseIosApplicationBundle extends IosAssetBundle {
 /// but it isn't actually executed. To generate something valid, we compile a trivial
 /// constant.
 Future<void> _createStubAppFramework(File outputFile, Environment environment,
-    Set<String> iosArchNames) async {
+    Set<String>? iosArchNames, String sdkRoot) async {
   try {
     outputFile.createSync(recursive: true);
   } on Exception catch (e) {
@@ -590,13 +591,12 @@ Future<void> _createStubAppFramework(File outputFile, Environment environment,
   static const int Moo = 88;
   ''');
 
-    final String sdkRoot = environment.defines[kSdkRoot];
-    final EnvironmentType environmentType = environmentTypeFromSdkroot(sdkRoot, fileSystem);
+    final EnvironmentType? environmentType = environmentTypeFromSdkroot(sdkRoot, fileSystem);
 
-    await globals.xcode.clang(<String>[
+    await globals.xcode!.clang(<String>[
       '-x',
       'c',
-      for (String arch in iosArchNames) ...<String>['-arch', arch],
+      for (String arch in iosArchNames ?? <String>{}) ...<String>['-arch', arch],
       stubSource.path,
       '-dynamiclib',
       '-fembed-bitcode-marker',
@@ -625,7 +625,7 @@ Future<void> _createStubAppFramework(File outputFile, Environment environment,
 }
 
 void _signFramework(Environment environment, String binaryPath, BuildMode buildMode) {
-  String codesignIdentity = environment.defines[kCodesignIdentity];
+  String? codesignIdentity = environment.defines[kCodesignIdentity];
   if (codesignIdentity == null || codesignIdentity.isEmpty) {
     codesignIdentity = '-';
   }
diff --git a/packages/flutter_tools/lib/src/build_system/targets/linux.dart b/packages/flutter_tools/lib/src/build_system/targets/linux.dart
index e9155d76ec4..549c48cc4d6 100644
--- a/packages/flutter_tools/lib/src/build_system/targets/linux.dart
+++ b/packages/flutter_tools/lib/src/build_system/targets/linux.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import '../../artifacts.dart';
 import '../../base/file_system.dart';
 import '../../build_info.dart';
@@ -49,7 +47,11 @@ class UnpackLinux extends Target {
 
   @override
   Future<void> build(Environment environment) async {
-    final BuildMode buildMode = getBuildModeForName(environment.defines[kBuildMode]);
+    final String? buildModeEnvironment = environment.defines[kBuildMode];
+    if (buildModeEnvironment == null) {
+      throw MissingDefineException(kBuildMode, name);
+    }
+    final BuildMode buildMode = getBuildModeForName(buildModeEnvironment);
     final String engineSourcePath = environment.artifacts
       .getArtifactPath(
         Artifact.linuxDesktopPath,
@@ -117,10 +119,11 @@ abstract class BundleLinuxAssets extends Target {
 
   @override
   Future<void> build(Environment environment) async {
-    if (environment.defines[kBuildMode] == null) {
+    final String? buildModeEnvironment = environment.defines[kBuildMode];
+    if (buildModeEnvironment == null) {
       throw MissingDefineException(kBuildMode, 'bundle_linux_assets');
     }
-    final BuildMode buildMode = getBuildModeForName(environment.defines[kBuildMode]);
+    final BuildMode buildMode = getBuildModeForName(buildModeEnvironment);
     final Directory outputDirectory = environment.outputDir
       .childDirectory('flutter_assets');
     if (!outputDirectory.existsSync()) {
diff --git a/packages/flutter_tools/lib/src/build_system/targets/macos.dart b/packages/flutter_tools/lib/src/build_system/targets/macos.dart
index 35f3b6066fc..53b3028829f 100644
--- a/packages/flutter_tools/lib/src/build_system/targets/macos.dart
+++ b/packages/flutter_tools/lib/src/build_system/targets/macos.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import '../../artifacts.dart';
 import '../../base/build.dart';
 import '../../base/file_system.dart';
@@ -46,10 +44,11 @@ abstract class UnpackMacOS extends Target {
 
   @override
   Future<void> build(Environment environment) async {
-    if (environment.defines[kBuildMode] == null) {
+    final String? buildModeEnvironment = environment.defines[kBuildMode];
+    if (buildModeEnvironment == null) {
       throw MissingDefineException(kBuildMode, 'unpack_macos');
     }
-    final BuildMode buildMode = getBuildModeForName(environment.defines[kBuildMode]);
+    final BuildMode buildMode = getBuildModeForName(buildModeEnvironment);
     final String basePath = environment.artifacts.getArtifactPath(Artifact.flutterMacOSFramework, mode: buildMode);
 
     final ProcessResult result = environment.processManager.runSync(<String>[
@@ -182,7 +181,7 @@ class DebugMacOSFramework extends Target {
 
     final Iterable<DarwinArch> darwinArchs = environment.defines[kDarwinArchs]
       ?.split(' ')
-      ?.map(getDarwinArchForName)
+      .map(getDarwinArchForName)
       ?? <DarwinArch>[DarwinArch.x86_64];
 
     final Iterable<String> darwinArchArguments =
@@ -193,7 +192,7 @@ class DebugMacOSFramework extends Target {
         ..writeAsStringSync(r'''
 static const int Moo = 88;
 ''');
-    final RunResult result = await globals.xcode.clang(<String>[
+    final RunResult result = await globals.xcode!.clang(<String>[
       '-x',
       'c',
       debugApp.path,
@@ -231,23 +230,28 @@ class CompileMacOSFramework extends Target {
 
   @override
   Future<void> build(Environment environment) async {
-    if (environment.defines[kBuildMode] == null) {
+    final String? buildModeEnvironment = environment.defines[kBuildMode];
+    if (buildModeEnvironment == null) {
       throw MissingDefineException(kBuildMode, 'compile_macos_framework');
     }
-    final BuildMode buildMode = getBuildModeForName(environment.defines[kBuildMode]);
+    final String? targetPlatformEnvironment = environment.defines[kTargetPlatform];
+    if (targetPlatformEnvironment == null) {
+      throw MissingDefineException(kTargetPlatform, 'kernel_snapshot');
+    }
+    final BuildMode buildMode = getBuildModeForName(buildModeEnvironment);
     if (buildMode == BuildMode.debug) {
       throw Exception('precompiled macOS framework only supported in release/profile builds.');
     }
     final String buildOutputPath = environment.buildDir.path;
-    final String codeSizeDirectory = environment.defines[kCodeSizeDirectory];
-    final String splitDebugInfo = environment.defines[kSplitDebugInfo];
+    final String? codeSizeDirectory = environment.defines[kCodeSizeDirectory];
+    final String? splitDebugInfo = environment.defines[kSplitDebugInfo];
     final bool dartObfuscation = environment.defines[kDartObfuscation] == 'true';
     final List<String> extraGenSnapshotOptions = decodeCommaSeparated(environment.defines, kExtraGenSnapshotOptions);
-    final TargetPlatform targetPlatform = getTargetPlatformForName(environment.defines[kTargetPlatform]);
+    final TargetPlatform targetPlatform = getTargetPlatformForName(targetPlatformEnvironment);
     final List<DarwinArch> darwinArchs = environment.defines[kDarwinArchs]
       ?.split(' ')
-      ?.map(getDarwinArchForName)
-      ?.toList()
+      .map(getDarwinArchForName)
+      .toList()
       ?? <DarwinArch>[DarwinArch.x86_64];
     if (targetPlatform != TargetPlatform.darwin) {
       throw Exception('compile_macos_framework is only supported for darwin TargetPlatform.');
@@ -256,7 +260,7 @@ class CompileMacOSFramework extends Target {
     final AOTSnapshotter snapshotter = AOTSnapshotter(
       fileSystem: environment.fileSystem,
       logger: environment.logger,
-      xcode: globals.xcode,
+      xcode: globals.xcode!,
       artifacts: environment.artifacts,
       processManager: environment.processManager
     );
@@ -353,10 +357,11 @@ abstract class MacOSBundleFlutterAssets extends Target {
 
   @override
   Future<void> build(Environment environment) async {
-    if (environment.defines[kBuildMode] == null) {
+    final String? buildModeEnvironment = environment.defines[kBuildMode];
+    if (buildModeEnvironment == null) {
       throw MissingDefineException(kBuildMode, 'compile_macos_framework');
     }
-    final BuildMode buildMode = getBuildModeForName(environment.defines[kBuildMode]);
+    final BuildMode buildMode = getBuildModeForName(buildModeEnvironment);
     final Directory frameworkRootDirectory = environment
         .outputDir
         .childDirectory('App.framework');
diff --git a/packages/flutter_tools/lib/src/build_system/targets/web.dart b/packages/flutter_tools/lib/src/build_system/targets/web.dart
index d28459c4cf1..462c22febbe 100644
--- a/packages/flutter_tools/lib/src/build_system/targets/web.dart
+++ b/packages/flutter_tools/lib/src/build_system/targets/web.dart
@@ -2,12 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:math';
 
 import 'package:crypto/crypto.dart';
-import 'package:meta/meta.dart';
 import 'package:package_config/package_config.dart';
 
 import '../../artifacts.dart';
@@ -22,6 +19,7 @@ import '../../globals.dart' as globals;
 import '../../project.dart';
 import '../build_system.dart';
 import '../depfile.dart';
+import '../exceptions.dart';
 import 'assets.dart';
 import 'localizations.dart';
 
@@ -64,7 +62,7 @@ const String kOfflineFirst = 'offline-first';
 const String kNoneWorker = 'none';
 
 /// Convert a [value] into a [ServiceWorkerStrategy].
-ServiceWorkerStrategy _serviceWorkerStrategyFromString(String value) {
+ServiceWorkerStrategy _serviceWorkerStrategyFromString(String? value) {
   switch (value) {
     case kNoneWorker:
       return ServiceWorkerStrategy.none;
@@ -97,7 +95,7 @@ class WebEntrypointTarget extends Target {
 
   @override
   Future<void> build(Environment environment) async {
-    final String targetFile = environment.defines[kTargetFile];
+    final String? targetFile = environment.defines[kTargetFile];
     final bool hasPlugins = environment.defines[kHasWebPlugins] == 'true';
     final Uri importUri = environment.fileSystem.file(targetFile).absolute.uri;
     // TODO(zanderso): support configuration of this file.
@@ -110,7 +108,7 @@ class WebEntrypointTarget extends Target {
     final LanguageVersion languageVersion = determineLanguageVersion(
       environment.fileSystem.file(targetFile),
       packageConfig[flutterProject.manifest.appName],
-      Cache.flutterRoot,
+      Cache.flutterRoot!,
     );
 
     // Use the PackageConfig to find the correct package-scheme import path
@@ -211,16 +209,21 @@ class Dart2JSTarget extends Target {
 
   @override
   Future<void> build(Environment environment) async {
-    final BuildMode buildMode = getBuildModeForName(environment.defines[kBuildMode]);
+    final String? buildModeEnvironment = environment.defines[kBuildMode];
+    if (buildModeEnvironment == null) {
+      throw MissingDefineException(kBuildMode, name);
+    }
+    final BuildMode buildMode = getBuildModeForName(buildModeEnvironment);
     final bool sourceMapsEnabled = environment.defines[kSourceMapsEnabled] == 'true';
     final bool nativeNullAssertions = environment.defines[kNativeNullAssertions] == 'true';
-    final String librariesSpec = (globals.artifacts.getHostArtifact(HostArtifact.flutterWebSdk) as Directory).childFile('libraries.json').path;
+    final Artifacts artifacts = globals.artifacts!;
+    final String librariesSpec = (artifacts.getHostArtifact(HostArtifact.flutterWebSdk) as Directory).childFile('libraries.json').path;
     final List<String> sharedCommandOptions = <String>[
-      globals.artifacts.getHostArtifact(HostArtifact.engineDartBinary).path,
+      artifacts.getHostArtifact(HostArtifact.engineDartBinary).path,
       '--disable-dart-dev',
-      globals.artifacts.getHostArtifact(HostArtifact.dart2jsSnapshot).path,
+      artifacts.getHostArtifact(HostArtifact.dart2jsSnapshot).path,
       '--libraries-spec=$librariesSpec',
-      ...?decodeCommaSeparated(environment.defines, kExtraFrontEndOptions),
+      ...decodeCommaSeparated(environment.defines, kExtraFrontEndOptions),
       if (nativeNullAssertions)
         '--native-null-assertions',
       if (buildMode == BuildMode.profile)
@@ -247,7 +250,7 @@ class Dart2JSTarget extends Target {
       throw Exception(_collectOutput(kernelResult));
     }
 
-    final String dart2jsOptimization = environment.defines[kDart2jsOptimization];
+    final String? dart2jsOptimization = environment.defines[kDart2jsOptimization];
     final File outputJSFile = environment.buildDir.childFile('main.dart.js');
     final bool csp = environment.defines[kCspMode] == 'true';
 
@@ -384,12 +387,11 @@ class WebReleaseBundle extends Target {
             "navigator.serviceWorker.register('flutter_service_worker.js')",
             "navigator.serviceWorker.register('flutter_service_worker.js?v=$randomHash')",
           );
-        if (resultString.contains(kBaseHrefPlaceholder) &&
-            environment.defines[kBaseHref] == null) {
+        final String? baseHref = environment.defines[kBaseHref];
+        if (resultString.contains(kBaseHrefPlaceholder) && baseHref == null) {
           resultString = resultString.replaceAll(kBaseHrefPlaceholder, '/');
-        } else if (resultString.contains(kBaseHrefPlaceholder) &&
-            environment.defines[kBaseHref] != null) {
-          resultString = resultString.replaceAll(kBaseHrefPlaceholder, environment.defines[kBaseHref]);
+        } else if (resultString.contains(kBaseHrefPlaceholder) && baseHref != null) {
+          resultString = resultString.replaceAll(kBaseHrefPlaceholder, baseHref);
         }
         outputFile.writeAsStringSync(resultString);
         continue;
@@ -549,7 +551,7 @@ class WebServiceWorker extends Target {
 String generateServiceWorker(
   Map<String, String> resources,
   List<String> coreBundle, {
-  @required ServiceWorkerStrategy serviceWorkerStrategy,
+  required ServiceWorkerStrategy serviceWorkerStrategy,
 }) {
   if (serviceWorkerStrategy == ServiceWorkerStrategy.none) {
     return '';
diff --git a/packages/flutter_tools/lib/src/build_system/targets/windows.dart b/packages/flutter_tools/lib/src/build_system/targets/windows.dart
index 12e4dc9a9c0..4eed095b1ca 100644
--- a/packages/flutter_tools/lib/src/build_system/targets/windows.dart
+++ b/packages/flutter_tools/lib/src/build_system/targets/windows.dart
@@ -2,10 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
-import 'package:meta/meta.dart';
-
 import '../../artifacts.dart';
 import '../../base/file_system.dart';
 import '../../build_info.dart';
@@ -68,7 +64,11 @@ class UnpackWindows extends Target {
 
   @override
   Future<void> build(Environment environment) async {
-    final BuildMode buildMode = getBuildModeForName(environment.defines[kBuildMode]);
+    final String? buildModeEnvironment = environment.defines[kBuildMode];
+    if (buildModeEnvironment == null) {
+      throw MissingDefineException(kBuildMode, name);
+    }
+    final BuildMode buildMode = getBuildModeForName(buildModeEnvironment);
     final String engineSourcePath = environment.artifacts
       .getArtifactPath(
         Artifact.windowsDesktopPath,
@@ -135,7 +135,11 @@ class UnpackWindowsUwp extends Target {
 
   @override
   Future<void> build(Environment environment) async {
-    final BuildMode buildMode = getBuildModeForName(environment.defines[kBuildMode]);
+    final String? buildModeEnvironment = environment.defines[kBuildMode];
+    if (buildModeEnvironment == null) {
+      throw MissingDefineException(kBuildMode, name);
+    }
+    final BuildMode buildMode = getBuildModeForName(buildModeEnvironment);
     final String engineSourcePath = environment.artifacts
       .getArtifactPath(
         Artifact.windowsUwpDesktopPath,
@@ -209,10 +213,11 @@ abstract class BundleWindowsAssets extends Target {
 
   @override
   Future<void> build(Environment environment) async {
-    if (environment.defines[kBuildMode] == null) {
+    final String? buildModeEnvironment = environment.defines[kBuildMode];
+    if (buildModeEnvironment == null) {
       throw MissingDefineException(kBuildMode, 'bundle_windows_assets');
     }
-    final BuildMode buildMode = getBuildModeForName(environment.defines[kBuildMode]);
+    final BuildMode buildMode = getBuildModeForName(buildModeEnvironment);
     final Directory outputDirectory = environment.outputDir
       .childDirectory('flutter_assets');
     if (!outputDirectory.existsSync()) {
@@ -255,7 +260,7 @@ abstract class BundleWindowsAssetsUwp extends BundleWindowsAssets {
 /// A wrapper for AOT compilation that copies app.so into the output directory.
 class WindowsAotBundle extends Target {
   /// Create a [WindowsAotBundle] wrapper for [aotTarget].
-  const WindowsAotBundle(this.aotTarget, {@required this.uwp});
+  const WindowsAotBundle(this.aotTarget, {required this.uwp});
 
   /// The [AotElfBase] subclass that produces the app.so.
   final AotElfBase aotTarget;
diff --git a/packages/flutter_tools/test/integration.shard/deferred_components_test.dart b/packages/flutter_tools/test/integration.shard/deferred_components_test.dart
index d96d852d34b..33fb485a13f 100644
--- a/packages/flutter_tools/test/integration.shard/deferred_components_test.dart
+++ b/packages/flutter_tools/test/integration.shard/deferred_components_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:archive/archive.dart';
 import 'package:file/file.dart';
 import 'package:file_testing/file_testing.dart';
@@ -15,8 +13,8 @@ import 'test_driver.dart';
 import 'test_utils.dart';
 
 void main() {
-  Directory tempDir;
-  FlutterRunTestDriver _flutter;
+  late Directory tempDir;
+  late FlutterRunTestDriver _flutter;
 
   setUp(() async {
     tempDir = createResolvedTempDirectorySync('run_test.');