diff --git a/dev/conductor/bin/conductor.dart b/dev/conductor/bin/conductor.dart
index 26243efe56..8513038c09 100644
--- a/dev/conductor/bin/conductor.dart
+++ b/dev/conductor/bin/conductor.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
-
 // See: https://github.com/flutter/flutter/wiki/Release-process
 
 import 'dart:io' as io;
diff --git a/dev/conductor/lib/clean.dart b/dev/conductor/lib/clean.dart
index 19075573b5..30324bc20d 100644
--- a/dev/conductor/lib/clean.dart
+++ b/dev/conductor/lib/clean.dart
@@ -2,11 +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 'package:args/args.dart';
 import 'package:args/command_runner.dart';
 import 'package:file/file.dart';
-import 'package:meta/meta.dart';
 import 'package:platform/platform.dart';
 
 import './globals.dart';
@@ -22,7 +20,7 @@ const String kStateOption = 'state-file';
 /// If the release was not completed, this command will abort the release.
 class CleanCommand extends Command<void> {
   CleanCommand({
-    @required this.checkouts,
+    required this.checkouts,
   })  : platform = checkouts.platform,
         fileSystem = checkouts.fileSystem,
         stdio = checkouts.stdio {
@@ -52,13 +50,14 @@ class CleanCommand extends Command<void> {
 
   @override
   void run() {
-    final File stateFile = checkouts.fileSystem.file(argResults[kStateOption]);
+    final ArgResults argumentResults = argResults!;
+    final File stateFile = checkouts.fileSystem.file(argumentResults[kStateOption]);
     if (!stateFile.existsSync()) {
       throw ConductorException(
           'No persistent state file found at ${stateFile.path}!');
     }
 
-    if (!(argResults[kYesFlag] as bool)) {
+    if (!(argumentResults[kYesFlag] as bool)) {
       stdio.printStatus(
         'Are you sure you want to clean up the persistent state file at\n'
         '${stateFile.path} (y/n)?',
diff --git a/dev/conductor/lib/next.dart b/dev/conductor/lib/next.dart
index 368b616111..cef5e89b67 100644
--- a/dev/conductor/lib/next.dart
+++ b/dev/conductor/lib/next.dart
@@ -2,11 +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 'package:args/command_runner.dart';
 import 'package:file/file.dart' show File;
-import 'package:meta/meta.dart' show required, visibleForTesting;
+import 'package:meta/meta.dart' show visibleForTesting;
 import './globals.dart';
 import './proto/conductor_state.pb.dart' as pb;
 import './proto/conductor_state.pbenum.dart';
@@ -21,7 +19,7 @@ const String kForceFlag = 'force';
 /// Command to proceed from one [pb.ReleasePhase] to the next.
 class NextCommand extends Command<void> {
   NextCommand({
-    @required this.checkouts,
+    required this.checkouts,
   }) {
     final String defaultPath = defaultStateFilePath(checkouts.platform);
     argParser.addOption(
@@ -51,10 +49,10 @@ class NextCommand extends Command<void> {
   @override
   void run() {
     runNext(
-      autoAccept: argResults[kYesFlag] as bool,
+      autoAccept: argResults![kYesFlag] as bool,
       checkouts: checkouts,
-      force: argResults[kForceFlag] as bool,
-      stateFile: checkouts.fileSystem.file(argResults[kStateOption]),
+      force: argResults![kForceFlag] as bool,
+      stateFile: checkouts.fileSystem.file(argResults![kStateOption]),
     );
   }
 }
@@ -77,10 +75,10 @@ bool prompt(String message, Stdio stdio) {
 
 @visibleForTesting
 void runNext({
-  @required bool autoAccept,
-  @required bool force,
-  @required Checkouts checkouts,
-  @required File stateFile,
+  required bool autoAccept,
+  required bool force,
+  required Checkouts checkouts,
+  required File stateFile,
 }) {
   final Stdio stdio = checkouts.stdio;
   const List<CherrypickState> finishedStates = <CherrypickState>[
@@ -341,7 +339,6 @@ void runNext({
       break;
     case pb.ReleasePhase.RELEASE_COMPLETED:
       throw ConductorException('This release is finished.');
-      break;
   }
   final ReleasePhase nextPhase = getNextPhase(state.currentPhase);
   stdio.printStatus('\nUpdating phase from ${state.currentPhase} to $nextPhase...\n');
diff --git a/dev/conductor/lib/proto/compile_proto.sh b/dev/conductor/lib/proto/compile_proto.sh
index b5e928d004..06cc5a145e 100755
--- a/dev/conductor/lib/proto/compile_proto.sh
+++ b/dev/conductor/lib/proto/compile_proto.sh
@@ -22,8 +22,8 @@ if ! type dart >/dev/null 2>&1; then
   exit 1
 fi
 
-# Pin protoc-gen-dart to pre-nullsafe version.
-dart pub global activate protoc_plugin 19.3.1
+# Use null-safe protoc_plugin
+dart pub global activate protoc_plugin 20.0.0
 
 protoc --dart_out="$DIR" --proto_path="$DIR" "$DIR/conductor_state.proto"
 
diff --git a/dev/conductor/lib/proto/conductor_state.pb.dart b/dev/conductor/lib/proto/conductor_state.pb.dart
index 8df5f2659f..7da9b37e4c 100644
--- a/dev/conductor/lib/proto/conductor_state.pb.dart
+++ b/dev/conductor/lib/proto/conductor_state.pb.dart
@@ -6,7 +6,7 @@
 //  Generated code. Do not modify.
 //  source: conductor_state.proto
 //
-// @dart = 2.7
+// @dart = 2.12
 // ignore_for_file: annotate_overrides,camel_case_types,unnecessary_const,non_constant_identifier_names,library_prefixes,unused_import,unused_shown_name,return_of_invalid_type,unnecessary_this,prefer_final_fields
 
 import 'dart:core' as $core;
@@ -30,8 +30,8 @@ class Remote extends $pb.GeneratedMessage {
 
   Remote._() : super();
   factory Remote({
-    $core.String name,
-    $core.String url,
+    $core.String? name,
+    $core.String? url,
   }) {
     final _result = create();
     if (name != null) {
@@ -62,7 +62,7 @@ class Remote extends $pb.GeneratedMessage {
   static $pb.PbList<Remote> createRepeated() => $pb.PbList<Remote>();
   @$core.pragma('dart2js:noInline')
   static Remote getDefault() => _defaultInstance ??= $pb.GeneratedMessage.$_defaultFor<Remote>(create);
-  static Remote _defaultInstance;
+  static Remote? _defaultInstance;
 
   @$pb.TagNumber(1)
   $core.String get name => $_getSZ(0);
@@ -106,9 +106,9 @@ class Cherrypick extends $pb.GeneratedMessage {
 
   Cherrypick._() : super();
   factory Cherrypick({
-    $core.String trunkRevision,
-    $core.String appliedRevision,
-    CherrypickState state,
+    $core.String? trunkRevision,
+    $core.String? appliedRevision,
+    CherrypickState? state,
   }) {
     final _result = create();
     if (trunkRevision != null) {
@@ -142,7 +142,7 @@ class Cherrypick extends $pb.GeneratedMessage {
   static $pb.PbList<Cherrypick> createRepeated() => $pb.PbList<Cherrypick>();
   @$core.pragma('dart2js:noInline')
   static Cherrypick getDefault() => _defaultInstance ??= $pb.GeneratedMessage.$_defaultFor<Cherrypick>(create);
-  static Cherrypick _defaultInstance;
+  static Cherrypick? _defaultInstance;
 
   @$pb.TagNumber(1)
   $core.String get trunkRevision => $_getSZ(0);
@@ -210,15 +210,15 @@ class Repository extends $pb.GeneratedMessage {
 
   Repository._() : super();
   factory Repository({
-    $core.String candidateBranch,
-    $core.String startingGitHead,
-    $core.String currentGitHead,
-    $core.String checkoutPath,
-    Remote upstream,
-    Remote mirror,
-    $core.Iterable<Cherrypick> cherrypicks,
-    $core.String dartRevision,
-    $core.String workingBranch,
+    $core.String? candidateBranch,
+    $core.String? startingGitHead,
+    $core.String? currentGitHead,
+    $core.String? checkoutPath,
+    Remote? upstream,
+    Remote? mirror,
+    $core.Iterable<Cherrypick>? cherrypicks,
+    $core.String? dartRevision,
+    $core.String? workingBranch,
   }) {
     final _result = create();
     if (candidateBranch != null) {
@@ -270,7 +270,7 @@ class Repository extends $pb.GeneratedMessage {
   static $pb.PbList<Repository> createRepeated() => $pb.PbList<Repository>();
   @$core.pragma('dart2js:noInline')
   static Repository getDefault() => _defaultInstance ??= $pb.GeneratedMessage.$_defaultFor<Repository>(create);
-  static Repository _defaultInstance;
+  static Repository? _defaultInstance;
 
   @$pb.TagNumber(1)
   $core.String get candidateBranch => $_getSZ(0);
@@ -407,15 +407,15 @@ class ConductorState extends $pb.GeneratedMessage {
 
   ConductorState._() : super();
   factory ConductorState({
-    $core.String releaseChannel,
-    $core.String releaseVersion,
-    Repository engine,
-    Repository framework,
-    $fixnum.Int64 createdDate,
-    $fixnum.Int64 lastUpdatedDate,
-    $core.Iterable<$core.String> logs,
-    ReleasePhase currentPhase,
-    $core.String conductorVersion,
+    $core.String? releaseChannel,
+    $core.String? releaseVersion,
+    Repository? engine,
+    Repository? framework,
+    $fixnum.Int64? createdDate,
+    $fixnum.Int64? lastUpdatedDate,
+    $core.Iterable<$core.String>? logs,
+    ReleasePhase? currentPhase,
+    $core.String? conductorVersion,
   }) {
     final _result = create();
     if (releaseChannel != null) {
@@ -468,7 +468,7 @@ class ConductorState extends $pb.GeneratedMessage {
   static $pb.PbList<ConductorState> createRepeated() => $pb.PbList<ConductorState>();
   @$core.pragma('dart2js:noInline')
   static ConductorState getDefault() => _defaultInstance ??= $pb.GeneratedMessage.$_defaultFor<ConductorState>(create);
-  static ConductorState _defaultInstance;
+  static ConductorState? _defaultInstance;
 
   @$pb.TagNumber(1)
   $core.String get releaseChannel => $_getSZ(0);
diff --git a/dev/conductor/lib/proto/conductor_state.pbenum.dart b/dev/conductor/lib/proto/conductor_state.pbenum.dart
index 0e14fba55c..dfdc5f7074 100644
--- a/dev/conductor/lib/proto/conductor_state.pbenum.dart
+++ b/dev/conductor/lib/proto/conductor_state.pbenum.dart
@@ -6,7 +6,7 @@
 //  Generated code. Do not modify.
 //  source: conductor_state.proto
 //
-// @dart = 2.7
+// @dart = 2.12
 // ignore_for_file: annotate_overrides,camel_case_types,unnecessary_const,non_constant_identifier_names,library_prefixes,unused_import,unused_shown_name,return_of_invalid_type,unnecessary_this,prefer_final_fields
 
 // ignore_for_file: UNDEFINED_SHOWN_NAME
@@ -40,7 +40,7 @@ class ReleasePhase extends $pb.ProtobufEnum {
   ];
 
   static final $core.Map<$core.int, ReleasePhase> _byValue = $pb.ProtobufEnum.initByValue(values);
-  static ReleasePhase valueOf($core.int value) => _byValue[value];
+  static ReleasePhase? valueOf($core.int value) => _byValue[value];
 
   const ReleasePhase._($core.int v, $core.String n) : super(v, n);
 }
@@ -63,7 +63,7 @@ class CherrypickState extends $pb.ProtobufEnum {
   ];
 
   static final $core.Map<$core.int, CherrypickState> _byValue = $pb.ProtobufEnum.initByValue(values);
-  static CherrypickState valueOf($core.int value) => _byValue[value];
+  static CherrypickState? valueOf($core.int value) => _byValue[value];
 
   const CherrypickState._($core.int v, $core.String n) : super(v, n);
 }
diff --git a/dev/conductor/lib/proto/conductor_state.pbjson.dart b/dev/conductor/lib/proto/conductor_state.pbjson.dart
index 55bdb97c42..6f16e23da6 100644
--- a/dev/conductor/lib/proto/conductor_state.pbjson.dart
+++ b/dev/conductor/lib/proto/conductor_state.pbjson.dart
@@ -6,7 +6,7 @@
 //  Generated code. Do not modify.
 //  source: conductor_state.proto
 //
-// @dart = 2.7
+// @dart = 2.12
 // ignore_for_file: annotate_overrides,camel_case_types,unnecessary_const,non_constant_identifier_names,library_prefixes,unused_import,unused_shown_name,return_of_invalid_type,unnecessary_this,prefer_final_fields,deprecated_member_use_from_same_package
 
 import 'dart:core' as $core;
diff --git a/dev/conductor/lib/proto/conductor_state.pbserver.dart b/dev/conductor/lib/proto/conductor_state.pbserver.dart
index f97847ad74..e1b793df47 100644
--- a/dev/conductor/lib/proto/conductor_state.pbserver.dart
+++ b/dev/conductor/lib/proto/conductor_state.pbserver.dart
@@ -6,7 +6,7 @@
 //  Generated code. Do not modify.
 //  source: conductor_state.proto
 //
-// @dart = 2.7
+// @dart = 2.12
 // ignore_for_file: annotate_overrides,camel_case_types,unnecessary_const,non_constant_identifier_names,library_prefixes,unused_import,unused_shown_name,return_of_invalid_type,unnecessary_this,prefer_final_fields,deprecated_member_use_from_same_package
 
 export 'conductor_state.pb.dart';
diff --git a/dev/conductor/lib/roll_dev.dart b/dev/conductor/lib/roll_dev.dart
index 4a5c4c3a2d..a5eaa504ed 100644
--- a/dev/conductor/lib/roll_dev.dart
+++ b/dev/conductor/lib/roll_dev.dart
@@ -111,7 +111,7 @@ bool rollDev({
   required FrameworkRepository repository,
 }) {
   final String remoteName = argResults[kRemoteName] as String;
-  final String level = argResults[kIncrement] as String;
+  final String? level = argResults[kIncrement] as String?;
   final String candidateBranch = argResults[kCandidateBranch] as String;
   final bool justPrint = argResults[kJustPrint] as bool;
   final bool autoApprove = argResults[kYes] as bool;
diff --git a/dev/conductor/lib/start.dart b/dev/conductor/lib/start.dart
index 31b3825704..932fd9bdfc 100644
--- a/dev/conductor/lib/start.dart
+++ b/dev/conductor/lib/start.dart
@@ -2,12 +2,10 @@
 // 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/args.dart';
 import 'package:args/command_runner.dart';
 import 'package:file/file.dart';
 import 'package:fixnum/fixnum.dart';
-import 'package:meta/meta.dart';
 import 'package:platform/platform.dart';
 import 'package:process/process.dart';
 
@@ -35,8 +33,8 @@ const String kStateOption = 'state-file';
 /// Command to print the status of the current Flutter release.
 class StartCommand extends Command<void> {
   StartCommand({
-    @required this.checkouts,
-    @required this.flutterRoot,
+    required this.checkouts,
+    required this.flutterRoot,
   })  : platform = checkouts.platform,
         processManager = checkouts.processManager,
         fileSystem = checkouts.fileSystem,
@@ -125,7 +123,7 @@ class StartCommand extends Command<void> {
   final Stdio stdio;
 
   /// Git revision for the currently running Conductor.
-  String conductorVersion;
+  late final String conductorVersion;
 
   @override
   String get name => 'start';
@@ -135,6 +133,7 @@ class StartCommand extends Command<void> {
 
   @override
   void run() {
+    final ArgResults argumentResults = argResults!;
     if (!platform.isMacOS && !platform.isLinux) {
       throw ConductorException(
         'Error! This tool is only supported on macOS and Linux',
@@ -142,64 +141,64 @@ class StartCommand extends Command<void> {
     }
 
     final File stateFile = checkouts.fileSystem.file(
-      getValueFromEnvOrArgs(kStateOption, argResults, platform.environment),
+      getValueFromEnvOrArgs(kStateOption, argumentResults, platform.environment),
     );
     if (stateFile.existsSync()) {
       throw ConductorException(
-          'Error! A persistent state file already found at ${argResults[kStateOption]}.\n\n'
+          'Error! A persistent state file already found at ${argResults![kStateOption]}.\n\n'
           'Run `conductor clean` to cancel a previous release.');
     }
     final String frameworkUpstream = getValueFromEnvOrArgs(
       kFrameworkUpstreamOption,
-      argResults,
+      argumentResults,
       platform.environment,
-    );
+    )!;
     final String frameworkMirror = getValueFromEnvOrArgs(
       kFrameworkMirrorOption,
-      argResults,
+      argumentResults,
       platform.environment,
-    );
+    )!;
     final String engineUpstream = getValueFromEnvOrArgs(
       kEngineUpstreamOption,
-      argResults,
+      argumentResults,
       platform.environment,
-    );
+    )!;
     final String engineMirror = getValueFromEnvOrArgs(
       kEngineMirrorOption,
-      argResults,
+      argumentResults,
       platform.environment,
-    );
+    )!;
     final String candidateBranch = getValueFromEnvOrArgs(
       kCandidateOption,
-      argResults,
+      argumentResults,
       platform.environment,
-    );
+    )!;
     final String releaseChannel = getValueFromEnvOrArgs(
       kReleaseOption,
-      argResults,
+      argumentResults,
       platform.environment,
-    );
+    )!;
     final List<String> frameworkCherrypickRevisions = getValuesFromEnvOrArgs(
       kFrameworkCherrypicksOption,
-      argResults,
+      argumentResults,
       platform.environment,
     );
     final List<String> engineCherrypickRevisions = getValuesFromEnvOrArgs(
       kEngineCherrypicksOption,
-      argResults,
+      argumentResults,
       platform.environment,
     );
-    final String dartRevision = getValueFromEnvOrArgs(
+    final String? dartRevision = getValueFromEnvOrArgs(
       kDartRevisionOption,
-      argResults,
+      argumentResults,
       platform.environment,
       allowNull: true,
     );
     final String incrementLetter = getValueFromEnvOrArgs(
       kIncrementOption,
-      argResults,
+      argumentResults,
       platform.environment,
-    );
+    )!;
 
     if (!releaseCandidateBranchRegex.hasMatch(candidateBranch)) {
       throw ConductorException(
@@ -270,7 +269,7 @@ class StartCommand extends Command<void> {
       cherrypicks: engineCherrypicks,
       dartRevision: dartRevision,
       upstream: pb.Remote(name: 'upstream', url: engine.upstreamRemote.url),
-      mirror: pb.Remote(name: 'mirror', url: engine.mirrorRemote.url),
+      mirror: pb.Remote(name: 'mirror', url: engine.mirrorRemote!.url),
     );
     final FrameworkRepository framework = FrameworkRepository(
       checkouts,
@@ -328,7 +327,7 @@ class StartCommand extends Command<void> {
       checkoutPath: framework.checkoutDirectory.path,
       cherrypicks: frameworkCherrypicks,
       upstream: pb.Remote(name: 'upstream', url: framework.upstreamRemote.url),
-      mirror: pb.Remote(name: 'mirror', url: framework.mirrorRemote.url),
+      mirror: pb.Remote(name: 'mirror', url: framework.mirrorRemote!.url),
     );
 
     state.currentPhase = ReleasePhase.APPLY_ENGINE_CHERRYPICKS;
@@ -344,10 +343,10 @@ class StartCommand extends Command<void> {
 
   // To minimize merge conflicts, sort the commits by rev-list order.
   List<String> _sortCherrypicks({
-    @required Repository repository,
-    @required List<String> cherrypicks,
-    @required String upstreamRef,
-    @required String releaseRef,
+    required Repository repository,
+    required List<String> cherrypicks,
+    required String upstreamRef,
+    required String releaseRef,
   }) {
     if (cherrypicks.isEmpty) {
       return cherrypicks;
diff --git a/dev/conductor/lib/state.dart b/dev/conductor/lib/state.dart
index c0482208e3..eb97d1a4a4 100644
--- a/dev/conductor/lib/state.dart
+++ b/dev/conductor/lib/state.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' show jsonDecode, jsonEncode;
 
 import 'package:file/file.dart' show File;
@@ -29,9 +27,12 @@ String luciConsoleLink(String channel, String groupName) {
 }
 
 String defaultStateFilePath(Platform platform) {
-  assert(platform.environment['HOME'] != null);
+  final String? home = platform.environment['HOME'];
+  if (home == null) {
+    throw ConductorException(r'Environment variable $HOME must be set!');
+  }
   return <String>[
-    platform.environment['HOME'],
+    home,
     kStateFileName,
   ].join(platform.pathSeparator);
 }
@@ -170,12 +171,13 @@ String phaseInstructions(pb.ConductorState state) {
 ///
 /// Will throw a [ConductorException] if [ReleasePhase.RELEASE_COMPLETED] is
 /// passed as an argument, as there is no next phase.
-ReleasePhase getNextPhase(ReleasePhase previousPhase) {
-  assert(previousPhase != null);
-  if (previousPhase == ReleasePhase.RELEASE_COMPLETED) {
+ReleasePhase getNextPhase(ReleasePhase currentPhase) {
+  assert(currentPhase != null);
+  final ReleasePhase? nextPhase = ReleasePhase.valueOf(currentPhase.value + 1);
+  if (nextPhase == null) {
     throw ConductorException('There is no next ReleasePhase!');
   }
-  return ReleasePhase.valueOf(previousPhase.value + 1);
+  return nextPhase;
 }
 
 void writeStateToFile(File file, pb.ConductorState state, List<String> logs) {
diff --git a/dev/conductor/lib/status.dart b/dev/conductor/lib/status.dart
index 0548ef58d3..9c5ad0a9ee 100644
--- a/dev/conductor/lib/status.dart
+++ b/dev/conductor/lib/status.dart
@@ -2,11 +2,8 @@
 // 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:file/file.dart';
-import 'package:meta/meta.dart';
 import 'package:platform/platform.dart';
 
 import './proto/conductor_state.pb.dart' as pb;
@@ -20,7 +17,7 @@ const String kStateOption = 'state-file';
 /// Command to print the status of the current Flutter release.
 class StatusCommand extends Command<void> {
   StatusCommand({
-    @required this.checkouts,
+    required this.checkouts,
   })  : platform = checkouts.platform,
         fileSystem = checkouts.fileSystem,
         stdio = checkouts.stdio {
@@ -51,16 +48,16 @@ class StatusCommand extends Command<void> {
 
   @override
   void run() {
-    final File stateFile = checkouts.fileSystem.file(argResults[kStateOption]);
+    final File stateFile = checkouts.fileSystem.file(argResults![kStateOption]);
     if (!stateFile.existsSync()) {
       stdio.printStatus(
-          'No persistent state file found at ${argResults[kStateOption]}.');
+          'No persistent state file found at ${argResults![kStateOption]}.');
       return;
     }
     final pb.ConductorState state = readStateFromFile(stateFile);
 
     stdio.printStatus(presentState(state));
-    if (argResults[kVerboseFlag] as bool) {
+    if (argResults![kVerboseFlag] as bool) {
       stdio.printStatus('\nLogs:');
       state.logs.forEach(stdio.printStatus);
     }
diff --git a/dev/conductor/test/clean_test.dart b/dev/conductor/test/clean_test.dart
index 55258b16d1..4bfdf82490 100644
--- a/dev/conductor/test/clean_test.dart
+++ b/dev/conductor/test/clean_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:conductor/clean.dart';
 import 'package:conductor/repository.dart';
@@ -19,32 +17,19 @@ void main() {
     const String flutterRoot = '/flutter';
     const String checkoutsParentDirectory = '$flutterRoot/dev/tools/';
 
-    MemoryFileSystem fileSystem;
-    FakePlatform platform;
-    TestStdio stdio;
-    FakeProcessManager processManager;
+    late MemoryFileSystem fileSystem;
+    late FakePlatform platform;
+    late TestStdio stdio;
+    late FakeProcessManager processManager;
+    late CommandRunner<void> runner;
 
     setUp(() {
       stdio = TestStdio();
       fileSystem = MemoryFileSystem.test();
-    });
-
-    tearDown(() {
-      // Ensure these don't get re-used between tests
-      stdio = null;
-      fileSystem = null;
-      processManager = null;
-      platform = null;
-    });
-
-    CommandRunner<void> createRunner({
-      List<FakeCommand> commands,
-      String operatingSystem,
-    }) {
-      operatingSystem ??= const LocalPlatform().operatingSystem;
+      final String operatingSystem = const LocalPlatform().operatingSystem;
       final String pathSeparator = operatingSystem == 'windows' ? r'\' : '/';
 
-      processManager = FakeProcessManager.list(commands ?? <FakeCommand>[]);
+      processManager = FakeProcessManager.empty();
       platform = FakePlatform(
         environment: <String, String>{'HOME': '/path/to/user/home'},
         pathSeparator: pathSeparator,
@@ -59,11 +44,10 @@ void main() {
       final CleanCommand command = CleanCommand(
         checkouts: checkouts,
       );
-      return CommandRunner<void>('clean-test', '')..addCommand(command);
-    }
+      runner = CommandRunner<void>('clean-test', '')..addCommand(command);
+    });
 
     test('throws if no state file found', () async {
-      final CommandRunner<void> runner = createRunner();
       const String stateFile = '/state-file.json';
 
       await expectLater(
@@ -80,7 +64,6 @@ void main() {
     });
 
     test('deletes state file', () async {
-      final CommandRunner<void> runner = createRunner();
       final File stateFile = fileSystem.file('/state-file.json');
       stateFile.writeAsStringSync('{}');
 
diff --git a/dev/conductor/test/common.dart b/dev/conductor/test/common.dart
index a397dc3afe..274d949aee 100644
--- a/dev/conductor/test/common.dart
+++ b/dev/conductor/test/common.dart
@@ -56,7 +56,7 @@ class TestStdio extends Stdio {
 
 class FakeArgResults implements ArgResults {
   FakeArgResults({
-    required String level,
+    required String? level,
     required String candidateBranch,
     String remote = 'upstream',
     bool justPrint = false,
diff --git a/dev/conductor/test/next_test.dart b/dev/conductor/test/next_test.dart
index caa7c773fa..e991dc4e07 100644
--- a/dev/conductor/test/next_test.dart
+++ b/dev/conductor/test/next_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:conductor/next.dart';
 import 'package:conductor/proto/conductor_state.pb.dart' as pb;
@@ -12,7 +10,6 @@ import 'package:conductor/repository.dart';
 import 'package:conductor/state.dart';
 import 'package:file/file.dart';
 import 'package:file/memory.dart';
-import 'package:meta/meta.dart';
 import 'package:platform/platform.dart';
 
 import './common.dart';
@@ -31,8 +28,8 @@ void main() {
     const String revision3 = '789aaa';
     const String releaseVersion = '1.2.0-3.0.pre';
     const String releaseChannel = 'beta';
-    MemoryFileSystem fileSystem;
-    TestStdio stdio;
+    late MemoryFileSystem fileSystem;
+    late TestStdio stdio;
     const String stateFile = '/state-file.json';
 
     setUp(() {
@@ -41,7 +38,7 @@ void main() {
     });
 
     CommandRunner<void> createRunner({
-      @required Checkouts checkouts,
+      required Checkouts checkouts,
     }) {
       final NextCommand command = NextCommand(
         checkouts: checkouts,
@@ -239,9 +236,9 @@ void main() {
     });
 
     group('CODESIGN_ENGINE_BINARIES to APPLY_FRAMEWORK_CHERRYPICKS', () {
-      pb.ConductorState state;
-      FakeProcessManager processManager;
-      FakePlatform platform;
+      late pb.ConductorState state;
+      late FakeProcessManager processManager;
+      late FakePlatform platform;
 
       setUp(() {
         state = pb.ConductorState(
@@ -341,9 +338,9 @@ void main() {
       const String frameworkCheckoutPath = '$checkoutsParentDirectory/framework';
       const String engineCheckoutPath = '$checkoutsParentDirectory/engine';
       const String oldEngineVersion = '000000001';
-      FakeProcessManager processManager;
-      FakePlatform platform;
-      pb.ConductorState state;
+      late FakeProcessManager processManager;
+      late FakePlatform platform;
+      late pb.ConductorState state;
 
       setUp(() {
         processManager = FakeProcessManager.empty();
@@ -640,8 +637,8 @@ void main() {
     group('PUBLISH_VERSION to PUBLISH_CHANNEL', () {
       const String remoteName = 'upstream';
       const String releaseVersion = '1.2.0-3.0.pre';
-      pb.ConductorState state;
-      FakePlatform platform;
+      late pb.ConductorState state;
+      late FakePlatform platform;
 
       setUp(() {
         state = pb.ConductorState(
@@ -766,8 +763,8 @@ void main() {
 
     group('PUBLISH_CHANNEL to VERIFY_RELEASE', () {
       const String remoteName = 'upstream';
-      pb.ConductorState state;
-      FakePlatform platform;
+      late pb.ConductorState state;
+      late FakePlatform platform;
 
       setUp(() {
         state = pb.ConductorState(
diff --git a/dev/conductor/test/roll_dev_test.dart b/dev/conductor/test/roll_dev_test.dart
index 86e507b518..acac496a92 100644
--- a/dev/conductor/test/roll_dev_test.dart
+++ b/dev/conductor/test/roll_dev_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:conductor/globals.dart';
 import 'package:conductor/repository.dart';
 import 'package:conductor/roll_dev.dart';
@@ -23,13 +21,13 @@ void main() {
     const String nextVersion = '1.2.0-2.0.pre';
     const String candidateBranch = 'flutter-1.2-candidate.2';
     const String checkoutsParentDirectory = '/path/to/directory/';
-    FakeArgResults fakeArgResults;
-    MemoryFileSystem fileSystem;
-    TestStdio stdio;
-    FrameworkRepository repo;
-    Checkouts checkouts;
-    FakePlatform platform;
-    FakeProcessManager processManager;
+    late FakeArgResults fakeArgResults;
+    late MemoryFileSystem fileSystem;
+    late TestStdio stdio;
+    late FrameworkRepository repo;
+    late Checkouts checkouts;
+    late FakePlatform platform;
+    late FakeProcessManager processManager;
 
     setUp(() {
       stdio = TestStdio();
diff --git a/dev/conductor/test/start_test.dart b/dev/conductor/test/start_test.dart
index 0279c8777c..1eb2826200 100644
--- a/dev/conductor/test/start_test.dart
+++ b/dev/conductor/test/start_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 'dart:convert' show jsonDecode;
 
 import 'package:args/command_runner.dart';
@@ -28,11 +26,11 @@ void main() {
     const String candidateBranch = 'flutter-1.2-candidate.3';
     const String releaseChannel = 'stable';
     const String revision = 'abcd1234';
-    Checkouts checkouts;
-    MemoryFileSystem fileSystem;
-    FakePlatform platform;
-    TestStdio stdio;
-    FakeProcessManager processManager;
+    late Checkouts checkouts;
+    late MemoryFileSystem fileSystem;
+    late FakePlatform platform;
+    late TestStdio stdio;
+    late FakeProcessManager processManager;
 
     setUp(() {
       stdio = TestStdio();
@@ -40,9 +38,9 @@ void main() {
     });
 
     CommandRunner<void> createRunner({
-      Map<String, String> environment,
-      String operatingSystem,
-      List<FakeCommand> commands,
+      Map<String, String>? environment,
+      String? operatingSystem,
+      List<FakeCommand>? commands,
     }) {
       operatingSystem ??= const LocalPlatform().operatingSystem;
       final String pathSeparator = operatingSystem == 'windows' ? r'\' : '/';
@@ -74,13 +72,6 @@ void main() {
       return CommandRunner<void>('codesign-test', '')..addCommand(command);
     }
 
-    tearDown(() {
-      // Ensure we don't re-use these between tests.
-      processManager = null;
-      checkouts = null;
-      platform = null;
-    });
-
     test('throws exception if run from Windows', () async {
       final CommandRunner<void> runner = createRunner(
         commands: <FakeCommand>[
@@ -251,7 +242,7 @@ void main() {
       );
 
       final String stateFilePath = fileSystem.path.join(
-        platform.environment['HOME'],
+        platform.environment['HOME']!,
         kStateFileName,
       );