Migrate devicelab framework code to null safety. (#86325)

(Attempt to reland #85993)

dev/devicelab/lib/command/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:args/command_runner.dart';
 
 import 'package:flutter_devicelab/framework/runner.dart';
@@ -64,19 +62,19 @@ class TestCommand extends Command<void> {
 
   @override
   Future<void> run() async {
-    final List<String> taskArgsRaw = argResults['task-args'] as List<String>;
+    final List<String> taskArgsRaw = argResults!['task-args'] as List<String>;
     // Prepend '--' to convert args to options when passed to task
     final List<String> taskArgs = taskArgsRaw.map((String taskArg) => '--$taskArg').toList();
     print(taskArgs);
     await runTasks(
-      <String>[argResults['task'] as String],
-      deviceId: argResults['device-id'] as String,
-      gitBranch: argResults['git-branch'] as String,
-      localEngine: argResults['local-engine'] as String,
-      localEngineSrcPath: argResults['local-engine-src-path'] as String,
-      luciBuilder: argResults['luci-builder'] as String,
-      resultsPath: argResults['results-file'] as String,
-      silent: argResults['silent'] as bool,
+      <String>[argResults!['task'] as String],
+      deviceId: argResults!['device-id'] as String?,
+      gitBranch: argResults!['git-branch'] as String?,
+      localEngine: argResults!['local-engine'] as String?,
+      localEngineSrcPath: argResults!['local-engine-src-path'] as String?,
+      luciBuilder: argResults!['luci-builder'] as String?,
+      resultsPath: argResults!['results-file'] as String?,
+      silent: (argResults!['silent'] as bool?) ?? false,
       taskArgs: taskArgs,
     );
   }

dev/devicelab/lib/command/upload_metrics.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:args/command_runner.dart';
 
 import '../framework/cocoon.dart';
@@ -25,8 +23,8 @@ class UploadMetricsCommand extends Command<void> {
 
   @override
   Future<void> run() async {
-    final String resultsPath = argResults['results-file'] as String;
-    final String serviceAccountTokenFile = argResults['service-account-token-file'] as String;
+    final String resultsPath = argResults!['results-file'] as String;
+    final String? serviceAccountTokenFile = argResults!['service-account-token-file'] as String?;
 
     final Cocoon cocoon = Cocoon(serviceAccountTokenPath: serviceAccountTokenFile);
     return cocoon.sendResultsPath(resultsPath);

dev/devicelab/lib/framework/ab.dart

@@ -2,10 +2,7 @@
 // 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' as math;
-import 'package:meta/meta.dart';
 
 import 'task_result.dart';
 
@@ -43,8 +40,8 @@ class ABTest {
   final String localEngine;
   final String taskName;
   final DateTime runStart;
-  DateTime _runEnd;
-  DateTime get runEnd => _runEnd;
+  DateTime? _runEnd;
+  DateTime? get runEnd => _runEnd;
 
   final Map<String, List<double>> _aResults;
   final Map<String, List<double>> _bResults;
@@ -91,15 +88,15 @@ class ABTest {
     kLocalEngineKeyName:       localEngine,
     kTaskNameKeyName:          taskName,
     kRunStartKeyName:          runStart.toIso8601String(),
-    kRunEndKeyName:            runEnd.toIso8601String(),
+    kRunEndKeyName:            runEnd!.toIso8601String(),
     kAResultsKeyName:          _aResults,
     kBResultsKeyName:          _bResults,
   };
 
-  static void updateColumnLengths(List<int> lengths, List<String> results) {
+  static void updateColumnLengths(List<int> lengths, List<String?> results) {
     for (int column = 0; column < lengths.length; column++) {
       if (results[column] != null) {
-        lengths[column] = math.max(lengths[column], results[column].length);
+        lengths[column] = math.max(lengths[column], results[column]?.length ?? 0);
       }
     }
   }
@@ -107,10 +104,10 @@ class ABTest {
   static void formatResult(StringBuffer buffer,
                            List<int> lengths,
                            List<FieldJustification> aligns,
-                           List<String> values) {
+                           List<String?> values) {
     for (int column = 0; column < lengths.length; column++) {
       final int len = lengths[column];
-      String value = values[column];
+      String? value = values[column];
       if (value == null) {
         value = ''.padRight(len);
       } else {
@@ -142,9 +139,9 @@ class ABTest {
     final Map<String, _ScoreSummary> summariesA = _summarize(_aResults);
     final Map<String, _ScoreSummary> summariesB = _summarize(_bResults);
 
-    final List<List<String>> tableRows = <List<String>>[
+    final List<List<String?>> tableRows = <List<String?>>[
       for (final String scoreKey in <String>{...summariesA.keys, ...summariesB.keys})
-        <String>[
+        <String?>[
           scoreKey,
           summariesA[scoreKey]?.averageString, summariesA[scoreKey]?.noiseString,
           summariesB[scoreKey]?.averageString, summariesB[scoreKey]?.noiseString,
@@ -167,7 +164,7 @@ class ABTest {
 
     final List<int> lengths = List<int>.filled(6, 0);
     updateColumnLengths(lengths, titles);
-    for (final List<String> row in tableRows) {
+    for (final List<String?> row in tableRows) {
       updateColumnLengths(lengths, row);
     }
 
@@ -177,7 +174,7 @@ class ABTest {
           FieldJustification.CENTER,
           ...alignments.skip(1),
         ], titles);
-    for (final List<String> row in tableRows) {
+    for (final List<String?> row in tableRows) {
       formatResult(buffer, lengths, alignments, row);
     }
 
@@ -192,7 +189,7 @@ class ABTest {
       buffer.writeln('$scoreKey:');
       buffer.write('  A:\t');
       if (_aResults.containsKey(scoreKey)) {
-        for (final double score in _aResults[scoreKey]) {
+        for (final double score in _aResults[scoreKey]!) {
           buffer.write('${score.toStringAsFixed(2)}\t');
         }
       } else {
@@ -202,7 +199,7 @@ class ABTest {
 
       buffer.write('  B:\t');
       if (_bResults.containsKey(scoreKey)) {
-        for (final double score in _bResults[scoreKey]) {
+        for (final double score in _bResults[scoreKey]!) {
           buffer.write('${score.toStringAsFixed(2)}\t');
         }
       } else {
@@ -232,8 +229,8 @@ class ABTest {
     );
 
     for (final String scoreKey in _allScoreKeys) {
-      final _ScoreSummary summaryA = summariesA[scoreKey];
-      final _ScoreSummary summaryB = summariesB[scoreKey];
+      final _ScoreSummary? summaryA = summariesA[scoreKey];
+      final _ScoreSummary? summaryB = summariesB[scoreKey];
       buffer.write('$scoreKey\t');
 
       if (summaryA != null) {
@@ -261,8 +258,8 @@ class ABTest {
 
 class _ScoreSummary {
   _ScoreSummary({
-    @required this.average,
-    @required this.noise,
+    required this.average,
+    required this.noise,
   });
 
   /// Average (arithmetic mean) of a series of values collected by a benchmark.
@@ -275,14 +272,14 @@ class _ScoreSummary {
   String get averageString => average.toStringAsFixed(2);
   String get noiseString => '(${_ratioToPercent(noise)})';
 
-  String improvementOver(_ScoreSummary other) {
+  String improvementOver(_ScoreSummary? other) {
     return other == null ? '' : '${(average / other.average).toStringAsFixed(2)}x';
   }
 }
 
 void _addResult(TaskResult result, Map<String, List<double>> results) {
-  for (final String scoreKey in result.benchmarkScoreKeys) {
-    final double score = (result.data[scoreKey] as num).toDouble();
+  for (final String scoreKey in result.benchmarkScoreKeys ?? <String>[]) {
+    final double score = (result.data![scoreKey] as num).toDouble();
     results.putIfAbsent(scoreKey, () => <double>[]).add(score);
   }
 }

dev/devicelab/lib/framework/apk_utils.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 'dart:io';
 
 import 'package:path/path.dart' as path;
@@ -106,7 +104,7 @@ bool hasMultipleOccurrences(String text, Pattern pattern) {
 
 /// The Android home directory.
 String get _androidHome {
-  final String androidHome = Platform.environment['ANDROID_HOME'] ??
+  final String? androidHome = Platform.environment['ANDROID_HOME'] ??
       Platform.environment['ANDROID_SDK_ROOT'];
   if (androidHome == null || androidHome.isEmpty) {
     throw Exception('Environment variable `ANDROID_SDK_ROOT` is not set.');
@@ -118,9 +116,9 @@ String get _androidHome {
 Future<String> _evalApkAnalyzer(
   List<String> args, {
   bool printStdout = false,
-  String workingDirectory,
+  String? workingDirectory,
 }) async {
-  final String javaHome = await findJavaHome();
+  final String? javaHome = await findJavaHome();
   if (javaHome == null || javaHome.isEmpty) {
     throw Exception('No JAVA_HOME set.');
   }
@@ -259,7 +257,7 @@ class FlutterProject {
   String get androidPath => path.join(rootPath, 'android');
   String get iosPath => path.join(rootPath, 'ios');
 
-  Future<void> addCustomBuildType(String name, {String initWith}) async {
+  Future<void> addCustomBuildType(String name, {required String initWith}) async {
     final File buildScript = File(
       path.join(androidPath, 'app', 'build.gradle'),
     );
@@ -276,7 +274,7 @@ android {
     ''');
   }
 
-  Future<void> addGlobalBuildType(String name, {String initWith}) async {
+  Future<void> addGlobalBuildType(String name, {required String initWith}) async {
     final File buildScript = File(
       path.join(androidPath, 'build.gradle'),
     );
@@ -360,11 +358,11 @@ flutter:
     pubspec.writeAsStringSync(newContents);
   }
 
-  Future<void> runGradleTask(String task, {List<String> options}) async {
+  Future<void> runGradleTask(String task, {List<String>? options}) async {
     return _runGradleTask(workingDirectory: androidPath, task: task, options: options);
   }
 
-  Future<ProcessResult> resultOfGradleTask(String task, {List<String> options}) {
+  Future<ProcessResult> resultOfGradleTask(String task, {List<String>? options}) {
     return _resultOfGradleTask(workingDirectory: androidPath, task: task, options: options);
   }
 
@@ -416,7 +414,11 @@ class FlutterModuleProject {
   String get rootPath => path.join(parent.path, name);
 }
 
-Future<void> _runGradleTask({String workingDirectory, String task, List<String> options}) async {
+Future<void> _runGradleTask({
+  required String workingDirectory,
+  required String task,
+  List<String>? options,
+}) async {
   final ProcessResult result = await _resultOfGradleTask(
       workingDirectory: workingDirectory,
       task: task,
@@ -431,10 +433,13 @@ Future<void> _runGradleTask({String workingDirectory, String task, List<String>
     throw 'Gradle exited with error';
 }
 
-Future<ProcessResult> _resultOfGradleTask({String workingDirectory, String task,
-    List<String> options}) async {
+Future<ProcessResult> _resultOfGradleTask({
+  required String workingDirectory,
+  required String task,
+  List<String>? options,
+}) async {
   section('Find Java');
-  final String javaHome = await findJavaHome();
+  final String? javaHome = await findJavaHome();
 
   if (javaHome == null)
     throw TaskResult.failure('Could not find Java');
@@ -465,7 +470,7 @@ Future<ProcessResult> _resultOfGradleTask({String workingDirectory, String task,
 }
 
 /// Returns [null] if target matches [expectedTarget], otherwise returns an error message.
-String validateSnapshotDependency(FlutterProject project, String expectedTarget) {
+String? validateSnapshotDependency(FlutterProject project, String expectedTarget) {
   final File snapshotBlob = File(
       path.join(project.rootPath, 'build', 'app', 'intermediates',
           'flutter', 'debug', 'flutter_build.d'));

dev/devicelab/lib/framework/cocoon.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 'dart:async';
 import 'dart:convert' show Encoding, json;
 import 'dart:io';
@@ -20,12 +18,12 @@ import 'utils.dart';
 typedef ProcessRunSync = ProcessResult Function(
   String,
   List<String>, {
-  Map<String, String> environment,
+  Map<String, String>? environment,
   bool includeParentEnvironment,
   bool runInShell,
-  Encoding stderrEncoding,
-  Encoding stdoutEncoding,
-  String workingDirectory,
+  Encoding? stderrEncoding,
+  Encoding? stdoutEncoding,
+  String? workingDirectory,
 });
 
 /// Class for test runner to interact with Flutter's infrastructure service, Cocoon.
@@ -34,8 +32,8 @@ typedef ProcessRunSync = ProcessResult Function(
 /// To retrieve these results, the test runner needs to send results back so the database can be updated.
 class Cocoon {
   Cocoon({
-    String serviceAccountTokenPath,
-    @visibleForTesting Client httpClient,
+    String? serviceAccountTokenPath,
+    @visibleForTesting Client? httpClient,
     @visibleForTesting this.fs = const LocalFileSystem(),
     @visibleForTesting this.processRunSync = Process.runSync,
     @visibleForTesting this.requestRetryLimit = 5,
@@ -58,7 +56,7 @@ class Cocoon {
   final int requestRetryLimit;
 
   String get commitSha => _commitSha ?? _readCommitSha();
-  String _commitSha;
+  String? _commitSha;
 
   /// Parse the local repo for the current running commit.
   String _readCommitSha() {
@@ -85,9 +83,9 @@ class Cocoon {
   /// Send [TaskResult] to Cocoon.
   // TODO(chillers): Remove when sendResultsPath is used in prod. https://github.com/flutter/flutter/issues/72457
   Future<void> sendTaskResult({
-    @required String builderName,
-    @required TaskResult result,
-    @required String gitBranch,
+    required String builderName,
+    required TaskResult result,
+    required String gitBranch,
   }) async {
     assert(builderName != null);
     assert(gitBranch != null);
@@ -109,16 +107,11 @@ class Cocoon {
 
   /// Write the given parameters into an update task request and store the JSON in [resultsPath].
   Future<void> writeTaskResultToFile({
-    @required String builderName,
-    @required String gitBranch,
-    @required TaskResult result,
-    @required String resultsPath,
+    String? builderName,
+    String? gitBranch,
+    required TaskResult result,
+    required String resultsPath,
   }) async {
-    assert(builderName != null);
-    assert(gitBranch != null);
-    assert(result != null);
-    assert(resultsPath != null);
-
     final Map<String, dynamic> updateRequest = _constructUpdateRequest(
       gitBranch: gitBranch,
       builderName: builderName,
@@ -134,9 +127,9 @@ class Cocoon {
   }
 
   Map<String, dynamic> _constructUpdateRequest({
-    @required String builderName,
-    @required TaskResult result,
-    @required String gitBranch,
+    String? builderName,
+    required TaskResult result,
+    String? gitBranch,
   }) {
     final Map<String, dynamic> updateRequest = <String, dynamic>{
       'CommitBranch': gitBranch,
@@ -151,12 +144,12 @@ class Cocoon {
 
     final List<String> validScoreKeys = <String>[];
     if (result.benchmarkScoreKeys != null) {
-      for (final String scoreKey in result.benchmarkScoreKeys) {
-        final Object score = result.data[scoreKey];
+      for (final String scoreKey in result.benchmarkScoreKeys!) {
+        final Object score = result.data![scoreKey] as Object;
         if (score is num) {
           // Convert all metrics to double, which provide plenty of precision
           // without having to add support for multiple numeric types in Cocoon.
-          result.data[scoreKey] = score.toDouble();
+          result.data![scoreKey] = score.toDouble();
           validScoreKeys.add(scoreKey);
         }
       }
@@ -195,15 +188,15 @@ class Cocoon {
 class AuthenticatedCocoonClient extends BaseClient {
   AuthenticatedCocoonClient(
     this._serviceAccountTokenPath, {
-    @visibleForTesting Client httpClient,
-    @visibleForTesting FileSystem filesystem,
+    @visibleForTesting Client? httpClient,
+    @visibleForTesting FileSystem? filesystem,
   })  : _delegate = httpClient ?? Client(),
         _fs = filesystem ?? const LocalFileSystem();
 
   /// Authentication token to have the ability to upload and record test results.
   ///
   /// This is intended to only be passed on automated runs on LUCI post-submit.
-  final String _serviceAccountTokenPath;
+  final String? _serviceAccountTokenPath;
 
   /// Underlying [HttpClient] to send requests to.
   final Client _delegate;
@@ -213,7 +206,7 @@ class AuthenticatedCocoonClient extends BaseClient {
 
   /// Value contained in the service account token file that can be used in http requests.
   String get serviceAccountToken => _serviceAccountToken ?? _readServiceAccountTokenFile();
-  String _serviceAccountToken;
+  String? _serviceAccountToken;
 
   /// Get [serviceAccountToken] from the given service account file.
   String _readServiceAccountTokenFile() {

dev/devicelab/lib/framework/framework.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 'dart:async';
 import 'dart:convert';
 import 'dart:developer';
@@ -64,8 +62,8 @@ class _TaskRunner {
   _TaskRunner(this.task) {
     registerExtension('ext.cocoonRunTask',
         (String method, Map<String, String> parameters) async {
-      final Duration taskTimeout = parameters.containsKey('timeoutInMinutes')
-        ? Duration(minutes: int.parse(parameters['timeoutInMinutes']))
+      final Duration? taskTimeout = parameters.containsKey('timeoutInMinutes')
+        ? Duration(minutes: int.parse(parameters['timeoutInMinutes']!))
         : null;
       // This is only expected to be passed in unit test runs so they do not
       // kill the Dart process that is running them and waste time running config.
@@ -82,7 +80,7 @@ class _TaskRunner {
 
   final TaskFunction task;
 
-  Future<Device/*?*/> _getWorkingDeviceIfAvailable() async {
+  Future<Device?> _getWorkingDeviceIfAvailable() async {
     try {
       return await devices.workingDevice;
     } on DeviceException {
@@ -91,8 +89,8 @@ class _TaskRunner {
   }
 
   // TODO(ianh): workaround for https://github.com/dart-lang/sdk/issues/23797
-  RawReceivePort _keepAlivePort;
-  Timer _startTaskTimeout;
+  RawReceivePort? _keepAlivePort;
+  Timer? _startTaskTimeout;
   bool _taskStarted = false;
 
   final Completer<TaskResult> _completer = Completer<TaskResult>();
@@ -102,7 +100,7 @@ class _TaskRunner {
   /// Signals that this task runner finished running the task.
   Future<TaskResult> get whenDone => _completer.future;
 
-  Future<TaskResult> run(Duration taskTimeout, {
+  Future<TaskResult> run(Duration? taskTimeout, {
     bool runFlutterConfig = true,
     bool runProcessCleanup = true,
   }) async {
@@ -110,7 +108,7 @@ class _TaskRunner {
       _taskStarted = true;
       print('Running task with a timeout of $taskTimeout.');
       final String exe = Platform.isWindows ? '.exe' : '';
-      Set<RunningProcessInfo> beforeRunningDartInstances;
+      late Set<RunningProcessInfo> beforeRunningDartInstances;
       if (runProcessCleanup) {
         section('Checking running Dart$exe processes');
         beforeRunningDartInstances = await getRunningProcesses(
@@ -136,7 +134,7 @@ class _TaskRunner {
           '--enable-windows-desktop',
           '--enable-linux-desktop',
           '--enable-web',
-          if (localEngine != null) ...<String>['--local-engine', localEngine],
+          if (localEngine != null) ...<String>['--local-engine', localEngine!],
         ], canFail: true);
         if (configResult != 0) {
           print('Failed to enable configuration, tasks may not run.');
@@ -145,12 +143,12 @@ class _TaskRunner {
         print('Skipping enabling configs for macOS, Linux, Windows, and Web');
       }
 
-      final Device/*?*/ device = await _getWorkingDeviceIfAvailable();
-      /*late*/ TaskResult result;
-      IOSink/*?*/ sink;
+      final Device? device = await _getWorkingDeviceIfAvailable();
+      late TaskResult result;
+      IOSink? sink;
       try {
         if (device != null && device.canStreamLogs && hostAgent.dumpDirectory != null) {
-          sink = File(path.join(hostAgent.dumpDirectory.path, '${device.deviceId}.log')).openWrite();
+          sink = File(path.join(hostAgent.dumpDirectory!.path, '${device.deviceId}.log')).openWrite();
           await device.startLoggingToSink(sink);
         }
 
@@ -212,7 +210,7 @@ class _TaskRunner {
       final File rebootFile = _rebootFile();
       int runCount;
       if (rebootFile.existsSync()) {
-        runCount = int.tryParse(rebootFile.readAsStringSync().trim());
+        runCount = int.tryParse(rebootFile.readAsStringSync().trim()) ?? 0;
       } else {
         runCount = 0;
       }
@@ -283,10 +281,10 @@ class _TaskRunner {
 
 File _rebootFile() {
   if (Platform.isLinux || Platform.isMacOS) {
-    return File(path.join(Platform.environment['HOME'], '.reboot-count'));
+    return File(path.join(Platform.environment['HOME']!, '.reboot-count'));
   }
   if (!Platform.isWindows) {
     throw StateError('Unexpected platform ${Platform.operatingSystem}');
   }
-  return File(path.join(Platform.environment['USERPROFILE'], '.reboot-count'));
+  return File(path.join(Platform.environment['USERPROFILE']!, '.reboot-count'));
 }

dev/devicelab/lib/framework/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 'dart:convert';
 
 import 'utils.dart';
@@ -42,17 +40,10 @@ Future<bool> containsBitcode(String pathToBinary) async {
   final List<String> lines = LineSplitter.split(loadCommands).toList();
   lines.asMap().forEach((int index, String line) {
     if (line.contains('segname __LLVM') && lines.length - index - 1 > 3) {
-      final String emptyBitcodeMarker = lines
+      emptyBitcodeMarkerFound |= lines
         .skip(index - 1)
         .take(4)
-        .firstWhere(
-          (String line) => line.contains(' size 0x0000000000000001'),
-          orElse: () => null,
-      );
-      if (emptyBitcodeMarker != null) {
-        emptyBitcodeMarkerFound = true;
-        return;
-      }
+        .any((String line) => line.contains(' size 0x0000000000000001'));
     }
   });
   return !emptyBitcodeMarkerFound;
@@ -79,16 +70,16 @@ Future<void> testWithNewIOSSimulator(
     workingDirectory: flutterDirectory.path,
   );
 
-  String iOSSimRuntime;
+  String? iOSSimRuntime;
 
   final RegExp iOSRuntimePattern = RegExp(r'iOS .*\) - (.*)');
 
   for (final String runtime in LineSplitter.split(availableRuntimes)) {
     // These seem to be in order, so allow matching multiple lines so it grabs
     // the last (hopefully latest) one.
-    final RegExpMatch iOSRuntimeMatch = iOSRuntimePattern.firstMatch(runtime);
+    final RegExpMatch? iOSRuntimeMatch = iOSRuntimePattern.firstMatch(runtime);
     if (iOSRuntimeMatch != null) {
-      iOSSimRuntime = iOSRuntimeMatch.group(1).trim();
+      iOSSimRuntime = iOSRuntimeMatch.group(1)!.trim();
       continue;
     }
   }

dev/devicelab/lib/framework/manifest.dart

@@ -2,17 +2,14 @@
 // 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:io';
 
-import 'package:meta/meta.dart';
 import 'package:yaml/yaml.dart';
 
 import 'utils.dart';
 
 /// Loads manifest data from `manifest.yaml` file or from [yaml], if present.
-Manifest loadTaskManifest([ String yaml ]) {
+Manifest loadTaskManifest([ String? yaml ]) {
   final dynamic manifestYaml = yaml == null
     ? loadYaml(file('manifest.yaml').readAsStringSync())
     : loadYamlNode(yaml);
@@ -32,13 +29,13 @@ class Manifest {
 /// A CI task.
 class ManifestTask {
   ManifestTask._({
-    @required this.name,
-    @required this.description,
-    @required this.stage,
-    @required this.requiredAgentCapabilities,
-    @required this.isFlaky,
-    @required this.timeoutInMinutes,
-    @required this.onLuci,
+    required this.name,
+    required this.description,
+    required this.stage,
+    required this.requiredAgentCapabilities,
+    required this.isFlaky,
+    required this.timeoutInMinutes,
+    required this.onLuci,
   }) {
     final String taskName = 'task "$name"';
     _checkIsNotBlank(name, 'Task name', taskName);
@@ -148,9 +145,9 @@ ManifestTask _validateAndParseTask(String taskName, dynamic taskYaml) {
     // ignore: avoid_dynamic_calls
     stage: taskYaml['stage'] as String,
     requiredAgentCapabilities: capabilities as List<String>,
-    isFlaky: isFlaky as bool ?? false,
+    isFlaky: isFlaky as bool,
     timeoutInMinutes: timeoutInMinutes as int,
-    onLuci: onLuci as bool ?? false,
+    onLuci: onLuci as bool,
   );
 }
 
@@ -161,7 +158,7 @@ List<String> _validateAndParseCapabilities(String taskName, dynamic capabilities
     final dynamic capability = capabilities[i];
     _checkType(capability is String, capability, 'required_agent_capabilities[$i]', 'string');
   }
-  return (capabilitiesYaml as List<dynamic>).cast<String>();
+  return capabilitiesYaml.cast<String>();
 }
 
 void _checkType(bool isValid, dynamic value, String variableName, String typeName) {

dev/devicelab/lib/framework/runner.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 'dart:async';
 import 'dart:convert';
 import 'dart:io';
@@ -21,13 +19,13 @@ Future<void> runTasks(
   List<String> taskNames, {
   bool exitOnFirstTestFailure = false,
   bool silent = false,
-  String deviceId,
-  String gitBranch,
-  String localEngine,
-  String localEngineSrcPath,
-  String luciBuilder,
-  String resultsPath,
-  List<String> taskArgs,
+  String? deviceId,
+  String? gitBranch,
+  String? localEngine,
+  String? localEngineSrcPath,
+  String? luciBuilder,
+  String? resultsPath,
+  List<String>? taskArgs,
 }) async {
   for (final String taskName in taskNames) {
     section('Running task "$taskName"');
@@ -44,7 +42,7 @@ Future<void> runTasks(
     print(const JsonEncoder.withIndent('  ').convert(result));
     section('Finished task "$taskName"');
 
-    if (resultsPath != null) {
+    if (resultsPath != null && gitBranch != null) {
       final Cocoon cocoon = Cocoon();
       await cocoon.writeTaskResultToFile(
         builderName: luciBuilder,
@@ -76,11 +74,11 @@ Future<void> runTasks(
 Future<TaskResult> runTask(
   String taskName, {
   bool silent = false,
-  String localEngine,
-  String localEngineSrcPath,
-  String deviceId,
-  List<String> taskArgs,
-  @visibleForTesting Map<String, String> isolateParams,
+  String? localEngine,
+  String? localEngineSrcPath,
+  String? deviceId,
+  List<String> ?taskArgs,
+  @visibleForTesting Map<String, String>? isolateParams,
 }) async {
   final String taskExecutable = 'bin/tasks/$taskName.dart';
 
@@ -117,7 +115,7 @@ Future<TaskResult> runTask(
       .transform<String>(const LineSplitter())
       .listen((String line) {
     if (!uri.isCompleted) {
-      final Uri serviceUri = parseServiceUri(line, prefix: 'Observatory listening on ');
+      final Uri? serviceUri = parseServiceUri(line, prefix: 'Observatory listening on ');
       if (serviceUri != null)
         uri.complete(serviceUri);
     }
@@ -139,7 +137,7 @@ Future<TaskResult> runTask(
       'ext.cocoonRunTask',
       args: isolateParams,
       isolateId: result.isolate.id,
-    )).json;
+    )).json!;
     final TaskResult taskResult = TaskResult.fromJson(taskResultJson);
     await runner.exitCode;
     return taskResult;
@@ -168,13 +166,13 @@ Future<ConnectionResult> _connectToRunnerIsolate(Uri vmServiceUri) async {
       // Look up the isolate.
       final VmService client = await vmServiceConnectUri(url);
       VM vm = await client.getVM();
-      while (vm.isolates.isEmpty) {
+      while (vm.isolates!.isEmpty) {
         await Future<void>.delayed(const Duration(seconds: 1));
         vm = await client.getVM();
       }
-      final IsolateRef isolate = vm.isolates.first;
+      final IsolateRef isolate = vm.isolates!.first;
       final Response response = await client.callServiceExtension('ext.cocoonRunnerReady', isolateId: isolate.id);
-      if (response.json['response'] != 'ready')
+      if (response.json!['response'] != 'ready')
         throw 'not ready yet';
       return ConnectionResult(client, isolate);
     } catch (error) {
@@ -193,7 +191,7 @@ class ConnectionResult {
 }
 
 /// The cocoon client sends an invalid VM service response, we need to intercept it.
-Future<VmService> vmServiceConnectUri(String wsUri, {Log log}) async {
+Future<VmService> vmServiceConnectUri(String wsUri, {Log? log}) async {
   final WebSocket socket = await WebSocket.connect(wsUri);
   final StreamController<dynamic> controller = StreamController<dynamic>();
   final Completer<dynamic> streamClosedCompleter = Completer<dynamic>();
@@ -207,7 +205,7 @@ Future<VmService> vmServiceConnectUri(String wsUri, {Log log}) async {
         controller.add(data);
       }
     },
-    onError: (dynamic err, StackTrace stackTrace) => controller.addError(err, stackTrace),
+    onError: (Object err, StackTrace stackTrace) => controller.addError(err, stackTrace),
     onDone: () => streamClosedCompleter.complete(),
   );
 

dev/devicelab/lib/framework/running_processes.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 'dart:io';
 
 import 'package:meta/meta.dart';
@@ -36,7 +34,7 @@ class RunningProcessInfo {
   }
 }
 
-Future<bool> killProcess(String pid, {ProcessManager processManager}) async {
+Future<bool> killProcess(String pid, {ProcessManager? processManager}) async {
   assert(pid != null, 'Must specify a pid to kill');
   processManager ??= const LocalProcessManager();
   ProcessResult result;
@@ -58,8 +56,8 @@ Future<bool> killProcess(String pid, {ProcessManager processManager}) async {
 }
 
 Stream<RunningProcessInfo> getRunningProcesses({
-  String processName,
-  ProcessManager processManager,
+  String? processName,
+  ProcessManager? processManager,
 }) {
   processManager ??= const LocalProcessManager();
   if (Platform.isWindows) {
@@ -69,7 +67,7 @@ Stream<RunningProcessInfo> getRunningProcesses({
 }
 
 @visibleForTesting
-Stream<RunningProcessInfo> windowsRunningProcesses(String processName) async* {
+Stream<RunningProcessInfo> windowsRunningProcesses(String? processName) async* {
   // PowerShell script to get the command line arguments and create time of
   // a process.
   // See: https://docs.microsoft.com/en-us/windows/desktop/cimwin32prov/win32-process
@@ -107,8 +105,8 @@ Iterable<RunningProcessInfo> processPowershellOutput(String output) sync* {
 
   const int processIdHeaderSize = 'ProcessId'.length;
   const int creationDateHeaderStart = processIdHeaderSize + 1;
-  int creationDateHeaderEnd;
-  int commandLineHeaderStart;
+  late int creationDateHeaderEnd;
+  late int commandLineHeaderStart;
   bool inTableBody = false;
   for (final String line in output.split('\n')) {
     if (line.startsWith('ProcessId')) {
@@ -160,7 +158,7 @@ Iterable<RunningProcessInfo> processPowershellOutput(String output) sync* {
 
 @visibleForTesting
 Stream<RunningProcessInfo> posixRunningProcesses(
-  String processName,
+  String? processName,
   ProcessManager processManager,
 ) async* {
   // Cirrus is missing this in Linux for some reason.
@@ -194,7 +192,7 @@ Stream<RunningProcessInfo> posixRunningProcesses(
 @visibleForTesting
 Iterable<RunningProcessInfo> processPsOutput(
   String output,
-  String processName,
+  String? processName,
 ) sync* {
   if (output == null) {
     return;
@@ -235,7 +233,7 @@ Iterable<RunningProcessInfo> processPsOutput(
     final String rawTime = line.substring(0, 24);
 
     final String year = rawTime.substring(20, 24);
-    final String month = months[rawTime.substring(4, 7)];
+    final String month = months[rawTime.substring(4, 7)]!;
     final String day = rawTime.substring(8, 10).replaceFirst(' ', '0');
     final String time = rawTime.substring(11, 19);
 

dev/devicelab/lib/framework/task_result.dart

@@ -41,7 +41,7 @@ class TaskResult {
     List<String> detailFiles = const <String>[],
   }) {
     return TaskResult.success(
-      json.decode(file.readAsStringSync()) as Map<String, dynamic>,
+      json.decode(file.readAsStringSync()) as Map<String, dynamic>?,
       benchmarkScoreKeys: benchmarkScoreKeys,
       detailFiles: detailFiles,
     );
@@ -53,14 +53,14 @@ class TaskResult {
     if (success) {
       final List<String> benchmarkScoreKeys = (json['benchmarkScoreKeys'] as List<dynamic>? ?? <String>[]).cast<String>();
       final List<String> detailFiles = (json['detailFiles'] as List<dynamic>? ?? <String>[]).cast<String>();
-      return TaskResult.success(json['data'] as Map<String, dynamic>,
+      return TaskResult.success(json['data'] as Map<String, dynamic>?,
         benchmarkScoreKeys: benchmarkScoreKeys,
         detailFiles: detailFiles,
-        message: json['reason'] as String,
+        message: json['reason'] as String?,
       );
     }
 
-    return TaskResult.failure(json['reason'] as String);
+    return TaskResult.failure(json['reason'] as String?);
   }
 
   /// Constructs an unsuccessful result.
@@ -88,7 +88,7 @@ class TaskResult {
   bool get failed => !succeeded;
 
   /// Explains the result in a human-readable format.
-  final String message;
+  final String? message;
 
   /// Serializes this task result to JSON format.
   ///
@@ -124,7 +124,7 @@ class TaskResult {
   }
 
   @override
-  String toString() => message;
+  String toString() => message ?? '';
 }
 
 class TaskResultCheckProcesses extends TaskResult {

dev/devicelab/lib/microbenchmarks.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 'dart:async';
 import 'dart:convert';
 import 'dart:io';

dev/devicelab/lib/tasks/analysis.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 'dart:io';
 
 import 'package:path/path.dart' as path;

dev/devicelab/lib/tasks/build_test_task.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 'dart:io';
 
 import 'package:args/args.dart';
@@ -18,10 +16,9 @@ import '../framework/utils.dart';
 abstract class BuildTestTask {
   BuildTestTask(this.args, {this.workingDirectory, this.runFlutterClean = true,}) {
     final ArgResults argResults = argParser.parse(args);
-    applicationBinaryPath = argResults[kApplicationBinaryPathOption] as String;
+    applicationBinaryPath = argResults[kApplicationBinaryPathOption] as String?;
     buildOnly = argResults[kBuildOnlyFlag] as bool;
     testOnly = argResults[kTestOnlyFlag] as bool;
-
   }
 
   static const String kApplicationBinaryPathOption = 'application-binary-path';
@@ -48,10 +45,10 @@ abstract class BuildTestTask {
   /// Path to a built application to use in [test].
   ///
   /// If not given, will default to child's expected location.
-  String applicationBinaryPath;
+  String? applicationBinaryPath;
 
   /// Where the test artifacts are stored, such as performance results.
-  final Directory workingDirectory;
+  final Directory? workingDirectory;
 
   /// Run Flutter build to create [applicationBinaryPath].
   Future<void> build() async {
@@ -93,7 +90,7 @@ abstract class BuildTestTask {
   ///
   /// Tasks can override to support default values. Otherwise, it will default
   /// to needing to be passed as an argument in the test runner.
-  String getApplicationBinaryPath() => applicationBinaryPath;
+  String? getApplicationBinaryPath() => applicationBinaryPath;
 
   /// Run this task.
   ///

dev/devicelab/lib/tasks/dart_plugin_registry_tests.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 'dart:async';
 import 'dart:convert';
 import 'dart:io';
@@ -15,8 +13,8 @@ import 'package:flutter_devicelab/framework/utils.dart';
 import 'package:path/path.dart' as path;
 
 TaskFunction dartPluginRegistryTest({
-  String deviceIdOverride,
-  Map<String, String> environment,
+  String? deviceIdOverride,
+  Map<String, String>? environment,
 }) {
   final Directory tempDir = Directory.systemTemp
       .createTempSync('flutter_devicelab_dart_plugin_test.');

dev/devicelab/lib/tasks/gallery.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 'dart:convert';
 import 'dart:io';
 import 'dart:math' as math;
@@ -55,9 +53,9 @@ class GalleryTransitionTest {
   final bool needFullTimeline;
   final String testFile;
   final String timelineSummaryFile;
-  final String timelineTraceFile;
-  final String transitionDurationFile;
-  final String driverFile;
+  final String? timelineTraceFile;
+  final String? transitionDurationFile;
+  final String? driverFile;
 
   Future<TaskResult> call() async {
     final Device device = await devices.workingDevice;
@@ -65,7 +63,7 @@ class GalleryTransitionTest {
     final String deviceId = device.deviceId;
     final Directory galleryDirectory = dir('${flutterDirectory.path}/dev/integration_tests/flutter_gallery');
     await inDirectory<void>(galleryDirectory, () async {
-      String applicationBinaryPath;
+      String? applicationBinaryPath;
       if (deviceOperatingSystem == DeviceOperatingSystem.android) {
         section('BUILDING APPLICATION');
         await flutter(

dev/devicelab/lib/tasks/hot_mode_tests.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 'dart:async';
 import 'dart:convert';
 import 'dart:io';
@@ -20,7 +18,7 @@ final Directory flutterGalleryDir = dir(path.join(flutterDirectory.path, 'dev/in
 const String kSourceLine = 'fontSize: (orientation == Orientation.portrait) ? 32.0 : 24.0';
 const String kReplacementLine = 'fontSize: (orientation == Orientation.portrait) ? 34.0 : 24.0';
 
-TaskFunction createHotModeTest({String deviceIdOverride, Map<String, String> environment}) {
+TaskFunction createHotModeTest({String? deviceIdOverride, Map<String, String>? environment}) {
   // This file is modified during the test and needs to be restored at the end.
   final File flutterFrameworkSource = file(path.join(
     flutterDirectory.path, 'packages/flutter/lib/src/widgets/framework.dart',
@@ -35,13 +33,13 @@ TaskFunction createHotModeTest({String deviceIdOverride, Map<String, String> env
     final File benchmarkFile = file(path.join(_editedFlutterGalleryDir.path, 'hot_benchmark.json'));
     rm(benchmarkFile);
     final List<String> options = <String>[
-      '--hot', '-d', deviceIdOverride, '--benchmark', '--resident',  '--no-android-gradle-daemon', '--no-publish-port', '--verbose',
+      '--hot', '-d', deviceIdOverride!, '--benchmark', '--resident',  '--no-android-gradle-daemon', '--no-publish-port', '--verbose',
     ];
     int hotReloadCount = 0;
-    Map<String, dynamic> smallReloadData;
-    Map<String, dynamic> mediumReloadData;
-    Map<String, dynamic> largeReloadData;
-    Map<String, dynamic> freshRestartReloadsData;
+    late Map<String, dynamic> smallReloadData;
+    late Map<String, dynamic> mediumReloadData;
+    late Map<String, dynamic> largeReloadData;
+    late Map<String, dynamic> freshRestartReloadsData;
 
 
     await inDirectory<void>(flutterDirectory, () async {
@@ -215,9 +213,9 @@ TaskFunction createHotModeTest({String deviceIdOverride, Map<String, String> env
   };
 }
 
-Future<Map<String, Object>> captureReloadData(
+Future<Map<String, dynamic>> captureReloadData(
   List<String> options,
-  Map<String, String> environment,
+  Map<String, String>? environment,
   File benchmarkFile,
   void Function(String, Process) onLine,
 ) async {
@@ -247,7 +245,7 @@ Future<Map<String, Object>> captureReloadData(
 
   await Future.wait<void>(<Future<void>>[stdoutDone.future, stderrDone.future]);
   await process.exitCode;
-  final Map<String, dynamic> result = json.decode(benchmarkFile.readAsStringSync()) as Map<String, dynamic>;
+  final Map<String, Object> result = json.decode(benchmarkFile.readAsStringSync()) as Map<String, Object>;
   benchmarkFile.deleteSync();
   return result;
 }

dev/devicelab/lib/tasks/integration_tests.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 '../framework/devices.dart';
 import '../framework/framework.dart';
 import '../framework/task_result.dart';

dev/devicelab/lib/tasks/microbenchmarks.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 'dart:async';
 import 'dart:io';
 

dev/devicelab/lib/tasks/new_gallery.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 'dart:io';
 
 import 'package:flutter_devicelab/tasks/perf_tests.dart';

dev/devicelab/lib/tasks/platform_channels_benchmarks.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 'dart:io' show Process, Directory;
 
 import 'package:flutter_devicelab/framework/devices.dart' as adb;

dev/devicelab/lib/tasks/plugin_tests.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 'dart:io';
 
 import 'package:flutter_devicelab/framework/framework.dart';
@@ -31,8 +29,8 @@ class PluginTest {
 
   final String buildTarget;
   final List<String> options;
-  final Map<String, String> pluginCreateEnvironment;
-  final Map<String, String> appCreateEnvironment;
+  final Map<String, String>? pluginCreateEnvironment;
+  final Map<String, String>? appCreateEnvironment;
 
   Future<TaskResult> call() async {
     final Directory tempDir =
@@ -77,7 +75,7 @@ class _FlutterProject {
 
   String get rootPath => path.join(parent.path, name);
 
-  Future<void> addPlugin(String plugin, {String pluginPath}) async {
+  Future<void> addPlugin(String plugin, {String? pluginPath}) async {
     final File pubspec = File(path.join(rootPath, 'pubspec.yaml'));
     String content = await pubspec.readAsString();
     final String dependency =
@@ -100,9 +98,9 @@ class _FlutterProject {
       List<String> options,
       String target,
       {
-        String name,
-        String template,
-        Map<String, String> environment,
+        required String name,
+        required String template,
+        Map<String, String>? environment,
       }) async {
     await inDirectory(directory, () async {
       await flutter(

dev/devicelab/lib/tasks/web_benchmarks.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 'dart:async';
 import 'dart:convert' show json;
 import 'dart:io' as io;
@@ -13,7 +11,6 @@ import 'package:flutter_devicelab/framework/browser.dart';
 import 'package:flutter_devicelab/framework/task_result.dart';
 import 'package:flutter_devicelab/framework/utils.dart';
 import 'package:logging/logging.dart';
-import 'package:meta/meta.dart';
 import 'package:path/path.dart' as path;
 import 'package:shelf/shelf.dart';
 import 'package:shelf/shelf_io.dart' as shelf_io;
@@ -23,7 +20,7 @@ import 'package:shelf_static/shelf_static.dart';
 const int benchmarkServerPort = 9999;
 const int chromeDebugPort = 10000;
 
-Future<TaskResult> runWebBenchmark({ @required bool useCanvasKit }) async {
+Future<TaskResult> runWebBenchmark({ required bool useCanvasKit }) async {
   // Reduce logging level. Otherwise, package:webkit_inspection_protocol is way too spammy.
   Logger.root.level = Level.INFO;
   final String macrobenchmarksDirectory = path.join(flutterDirectory.path, 'dev', 'benchmarks', 'macrobenchmarks');
@@ -38,17 +35,17 @@ Future<TaskResult> runWebBenchmark({ @required bool useCanvasKit }) async {
     ]);
     final Completer<List<Map<String, dynamic>>> profileData = Completer<List<Map<String, dynamic>>>();
     final List<Map<String, dynamic>> collectedProfiles = <Map<String, dynamic>>[];
-    List<String> benchmarks;
-    Iterator<String> benchmarkIterator;
+    List<String>? benchmarks;
+    late Iterator<String> benchmarkIterator;
 
     // This future fixes a race condition between the web-page loading and
     // asking to run a benchmark, and us connecting to Chrome's DevTools port.
     // Sometime one wins. Other times, the other wins.
-    Future<Chrome> whenChromeIsReady;
-    Chrome chrome;
-    io.HttpServer server;
+    Future<Chrome>? whenChromeIsReady;
+    Chrome? chrome;
+    late io.HttpServer server;
     Cascade cascade = Cascade();
-    List<Map<String, dynamic>> latestPerformanceTrace;
+    List<Map<String, dynamic>>? latestPerformanceTrace;
     cascade = cascade.add((Request request) async {
       try {
         chrome ??= await whenChromeIsReady;
@@ -66,7 +63,7 @@ Future<TaskResult> runWebBenchmark({ @required bool useCanvasKit }) async {
 
           // Trace data is null when the benchmark is not frame-based, such as RawRecorder.
           if (latestPerformanceTrace != null) {
-            final BlinkTraceSummary traceSummary = BlinkTraceSummary.fromJson(latestPerformanceTrace);
+            final BlinkTraceSummary traceSummary = BlinkTraceSummary.fromJson(latestPerformanceTrace!)!;
             profile['totalUiFrame.average'] = traceSummary.averageTotalUIFrameTime.inMicroseconds;
             profile['scoreKeys'] ??= <dynamic>[]; // using dynamic for consistency with JSON
             (profile['scoreKeys'] as List<dynamic>).add('totalUiFrame.average');
@@ -76,10 +73,10 @@ Future<TaskResult> runWebBenchmark({ @required bool useCanvasKit }) async {
           return Response.ok('Profile received');
         } else if (request.requestedUri.path.endsWith('/start-performance-tracing')) {
           latestPerformanceTrace = null;
-          await chrome.beginRecordingPerformance(request.requestedUri.queryParameters['label']);
+          await chrome!.beginRecordingPerformance(request.requestedUri.queryParameters['label']!);
           return Response.ok('Started performance tracing');
         } else if (request.requestedUri.path.endsWith('/stop-performance-tracing')) {
-          latestPerformanceTrace = await chrome.endRecordingPerformance();
+          latestPerformanceTrace = await chrome!.endRecordingPerformance();
           return Response.ok('Stopped performance tracing');
         } else if (request.requestedUri.path.endsWith('/on-error')) {
           final Map<String, dynamic> errorDetails = json.decode(await request.readAsString()) as Map<String, dynamic>;
@@ -90,7 +87,7 @@ Future<TaskResult> runWebBenchmark({ @required bool useCanvasKit }) async {
         } else if (request.requestedUri.path.endsWith('/next-benchmark')) {
           if (benchmarks == null) {
             benchmarks = (json.decode(await request.readAsString()) as List<dynamic>).cast<String>();
-            benchmarkIterator = benchmarks.iterator;
+            benchmarkIterator = benchmarks!.iterator;
           }
           if (benchmarkIterator.moveNext()) {
             final String nextBenchmark = benchmarkIterator.current;
@@ -186,7 +183,7 @@ Future<TaskResult> runWebBenchmark({ @required bool useCanvasKit }) async {
       }
       return TaskResult.success(taskResult, benchmarkScoreKeys: benchmarkScoreKeys);
     } finally {
-      unawaited(server?.close());
+      unawaited(server.close());
       chrome?.stop();
     }
   });

dev/devicelab/lib/tasks/web_dev_mode_tests.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 'dart:async';
 import 'dart:convert';
 import 'dart:io';