diff --git a/packages/flutter_tools/lib/src/runner/flutter_command.dart b/packages/flutter_tools/lib/src/runner/flutter_command.dart
index 4b3c8bea51..2bed8a461b 100644
--- a/packages/flutter_tools/lib/src/runner/flutter_command.dart
+++ b/packages/flutter_tools/lib/src/runner/flutter_command.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/args.dart';
 import 'package:args/command_runner.dart';
 import 'package:file/file.dart';
@@ -70,7 +68,7 @@ class FlutterCommandResult {
   /// Optional data that can be appended to the timing event.
   /// https://developers.google.com/analytics/devguides/collection/analyticsjs/field-reference#timingLabel
   /// Do not add PII.
-  final List<String> timingLabelParts;
+  final List<String>? timingLabelParts;
 
   /// Optional epoch time when the command's non-interactive wait time is
   /// complete during the command's execution. Use to measure user perceivable
@@ -78,7 +76,7 @@ class FlutterCommandResult {
   ///
   /// [FlutterCommand] will automatically measure and report the command's
   /// complete time if not overridden.
-  final DateTime endTimeOverride;
+  final DateTime? endTimeOverride;
 
   @override
   String toString() {
@@ -92,7 +90,6 @@ class FlutterCommandResult {
       case ExitStatus.killed:
         return 'killed';
     }
-    return null; // dead code, remove with null safety migration
   }
 }
 
@@ -131,7 +128,7 @@ abstract class FlutterCommand extends Command<void> {
   /// The currently executing command (or sub-command).
   ///
   /// Will be `null` until the top-most command has begun execution.
-  static FlutterCommand get current => context.get<FlutterCommand>();
+  static FlutterCommand? get current => context.get<FlutterCommand>();
 
   /// The option name for a custom observatory port.
   static const String observatoryPortOption = 'observatory-port';
@@ -168,7 +165,7 @@ abstract class FlutterCommand extends Command<void> {
   );
 
   @override
-  FlutterCommandRunner get runner => super.runner as FlutterCommandRunner;
+  FlutterCommandRunner? get runner => super.runner as FlutterCommandRunner?;
 
   bool _requiresPubspecYaml = false;
 
@@ -197,7 +194,7 @@ abstract class FlutterCommand extends Command<void> {
     _requiresPubspecYaml = true;
   }
 
-  void usesWebOptions({ @required bool verboseHelp }) {
+  void usesWebOptions({ required bool verboseHelp }) {
     argParser.addOption('web-hostname',
       defaultsTo: 'localhost',
       help:
@@ -208,7 +205,6 @@ abstract class FlutterCommand extends Command<void> {
       hide: !verboseHelp,
     );
     argParser.addOption('web-port',
-      defaultsTo: null,
       help: 'The host port to serve the web application from. If not provided, the tool '
         'will select a random open port on the host.',
       hide: !verboseHelp,
@@ -241,13 +237,11 @@ abstract class FlutterCommand extends Command<void> {
       hide: !verboseHelp,
     );
     argParser.addFlag('web-allow-expose-url',
-      defaultsTo: false,
       help: 'Enables daemon-to-editor requests (app.exposeUrl) for exposing URLs '
         'when running on remote machines.',
       hide: !verboseHelp,
     );
     argParser.addFlag('web-run-headless',
-      defaultsTo: false,
       help: 'Launches the browser in headless mode. Currently only Chrome '
         'supports this option.',
       hide: !verboseHelp,
@@ -278,11 +272,12 @@ abstract class FlutterCommand extends Command<void> {
   }
 
   String get targetFile {
-    if (argResults.wasParsed('target')) {
-      return stringArg('target');
+    if (argResults?.wasParsed('target') == true) {
+      return stringArg('target')!;
     }
-    if (argResults.rest.isNotEmpty) {
-      return argResults.rest.first;
+    final List<String>? rest = argResults?.rest;
+    if (rest != null && rest.isNotEmpty) {
+      return rest.first;
     }
     return bundle.defaultMainPath;
   }
@@ -290,12 +285,12 @@ abstract class FlutterCommand extends Command<void> {
   /// Path to the Dart's package config file.
   ///
   /// This can be overridden by some of its subclasses.
-  String get packagesPath => globalResults['packages'] as String;
+  String? get packagesPath => globalResults?['packages'] as String?;
 
   /// The value of the `--filesystem-scheme` argument.
   ///
   /// This can be overridden by some of its subclasses.
-  String get fileSystemScheme =>
+  String? get fileSystemScheme =>
     argParser.options.containsKey(FlutterOptions.kFileSystemScheme)
           ? stringArg(FlutterOptions.kFileSystemScheme)
           : null;
@@ -303,7 +298,7 @@ abstract class FlutterCommand extends Command<void> {
   /// The values of the `--filesystem-root` argument.
   ///
   /// This can be overridden by some of its subclasses.
-  List<String> get fileSystemRoots =>
+  List<String>? get fileSystemRoots =>
     argParser.options.containsKey(FlutterOptions.kFileSystemRoot)
           ? stringsArg(FlutterOptions.kFileSystemRoot)
           : null;
@@ -320,7 +315,7 @@ abstract class FlutterCommand extends Command<void> {
   ///
   /// The `hide` argument indicates whether or not to hide these options when
   /// the user asks for help.
-  void usesFilesystemOptions({ @required bool hide }) {
+  void usesFilesystemOptions({ required bool hide }) {
     argParser
       ..addOption('output-dill',
         hide: hide,
@@ -342,7 +337,7 @@ abstract class FlutterCommand extends Command<void> {
   }
 
   /// Adds options for connecting to the Dart VM observatory port.
-  void usesPortOptions({ @required bool verboseHelp }) {
+  void usesPortOptions({ required bool verboseHelp }) {
     argParser.addOption(observatoryPortOption,
         help: '(deprecated; use host-vmservice-port instead) '
               'Listen to the given port for an observatory debugger connection.\n'
@@ -364,7 +359,7 @@ abstract class FlutterCommand extends Command<void> {
     _usesPortOption = true;
   }
 
-  void addDevToolsOptions({@required bool verboseHelp}) {
+  void addDevToolsOptions({required bool verboseHelp}) {
     argParser.addFlag(
       kEnableDevTools,
       hide: !verboseHelp,
@@ -382,7 +377,7 @@ abstract class FlutterCommand extends Command<void> {
     );
   }
 
-  void addDdsOptions({@required bool verboseHelp}) {
+  void addDdsOptions({required bool verboseHelp}) {
     argParser.addOption('dds-port',
       help: 'When this value is provided, the Dart Development Service (DDS) will be '
             'bound to the provided port.\n'
@@ -407,55 +402,61 @@ abstract class FlutterCommand extends Command<void> {
     );
   }
 
-  bool _ddsEnabled;
-  bool get enableDds {
-    if (_ddsEnabled == null) {
-      if (argResults.wasParsed('disable-dds')) {
-        if (argResults.wasParsed('dds')) {
-          throwToolExit('The "--[no-]dds" and "--[no-]disable-dds" arguments are mutually exclusive. Only specify "--[no-]dds".');
-        }
-        _ddsEnabled = !boolArg('disable-dds');
-        // TODO(ianh): enable the following code once google3 is migrated away from --disable-dds (and add test to flutter_command_test.dart)
-        if (false) { // ignore: dead_code
-          if (_ddsEnabled) {
-            globals.printError('${globals.logger.terminal.warningMark} The "--no-disable-dds" argument is deprecated and redundant, and should be omitted.');
-          } else {
-            globals.printError('${globals.logger.terminal.warningMark} The "--disable-dds" argument is deprecated. Use "--no-dds" instead.');
-          }
-        }
-      } else {
-        _ddsEnabled = boolArg('dds');
+  late final bool enableDds = () {
+    bool ddsEnabled = false;
+    if (argResults?.wasParsed('disable-dds') == true) {
+      if (argResults?.wasParsed('dds') == true) {
+        throwToolExit(
+            'The "--[no-]dds" and "--[no-]disable-dds" arguments are mutually exclusive. Only specify "--[no-]dds".');
       }
+      ddsEnabled = !boolArg('disable-dds');
+      // TODO(ianh): enable the following code once google3 is migrated away from --disable-dds (and add test to flutter_command_test.dart)
+      if (false) { // ignore: dead_code
+        if (ddsEnabled) {
+          globals.printError('${globals.logger.terminal
+              .warningMark} The "--no-disable-dds" argument is deprecated and redundant, and should be omitted.');
+        } else {
+          globals.printError('${globals.logger.terminal
+              .warningMark} The "--disable-dds" argument is deprecated. Use "--no-dds" instead.');
+        }
+      }
+    } else {
+      ddsEnabled = boolArg('dds');
     }
-    return _ddsEnabled;
-  }
+    return ddsEnabled;
+  }();
 
-  bool get _hostVmServicePortProvided => argResults.wasParsed('observatory-port') ||
-                                         argResults.wasParsed('host-vmservice-port');
+  bool get _hostVmServicePortProvided => argResults?.wasParsed('observatory-port') == true ||
+                                         argResults?.wasParsed('host-vmservice-port') == true;
 
   int _tryParseHostVmservicePort() {
+    final String? observatoryPort = stringArg('observatory-port');
+    final String? hostPort = stringArg('host-vmservice-port');
+    if (observatoryPort == null && hostPort == null) {
+      throwToolExit('Invalid port for `--observatory-port/--host-vmservice-port`');
+    }
     try {
-      return int.parse(stringArg('observatory-port') ?? stringArg('host-vmservice-port'));
+      return int.parse((observatoryPort ?? hostPort)!);
     } on FormatException catch (error) {
       throwToolExit('Invalid port for `--observatory-port/--host-vmservice-port`: $error');
     }
   }
 
   int get ddsPort {
-    if (!argResults.wasParsed('dds-port') && _hostVmServicePortProvided) {
+    if (argResults?.wasParsed('dds-port') != true && _hostVmServicePortProvided) {
       // If an explicit DDS port is _not_ provided, use the host-vmservice-port for DDS.
       return _tryParseHostVmservicePort();
-    } else if (argResults.wasParsed('dds-port')) {
+    } else if (argResults?.wasParsed('dds-port') == true) {
       // If an explicit DDS port is provided, use dds-port for DDS.
-      return int.tryParse(stringArg('dds-port')) ?? 0;
+      return int.tryParse(stringArg('dds-port')!) ?? 0;
     }
     // Otherwise, DDS can bind to a random port.
     return 0;
   }
 
-  Uri get devToolsServerAddress {
-    if (argResults.wasParsed(kDevToolsServerAddress)) {
-      final Uri uri = Uri.tryParse(stringArg(kDevToolsServerAddress));
+  Uri? get devToolsServerAddress {
+    if (argResults?.wasParsed(kDevToolsServerAddress) == true) {
+      final Uri? uri = Uri.tryParse(stringArg(kDevToolsServerAddress)!);
       if (uri != null && uri.host.isNotEmpty && uri.port != 0) {
         return uri;
       }
@@ -470,19 +471,19 @@ abstract class FlutterCommand extends Command<void> {
   /// specified.
   ///
   /// If no port is set, returns null.
-  int get hostVmservicePort {
+  int? get hostVmservicePort {
     if (!_usesPortOption || !_hostVmServicePortProvided) {
       return null;
     }
-    if (argResults.wasParsed('observatory-port') &&
-        argResults.wasParsed('host-vmservice-port')) {
+    if (argResults?.wasParsed('observatory-port') == true &&
+        argResults?.wasParsed('host-vmservice-port') == true) {
       throwToolExit('Only one of "--observatory-port" and '
         '"--host-vmservice-port" may be specified.');
     }
     // If DDS is enabled and no explicit DDS port is provided, use the
     // host-vmservice-port for DDS instead and bind the VM service to a random
     // port.
-    if (enableDds && !argResults.wasParsed('dds-port')) {
+    if (enableDds && argResults?.wasParsed('dds-port') != true) {
       return null;
     }
     return _tryParseHostVmservicePort();
@@ -491,12 +492,13 @@ abstract class FlutterCommand extends Command<void> {
   /// Gets the vmservice port provided to in the 'device-vmservice-port' option.
   ///
   /// If no port is set, returns null.
-  int get deviceVmservicePort {
-    if (!_usesPortOption || argResults['device-vmservice-port'] == null) {
+  int? get deviceVmservicePort {
+    final String? devicePort = stringArg('device-vmservice-port');
+    if (!_usesPortOption || devicePort == null) {
       return null;
     }
     try {
-      return int.parse(stringArg('device-vmservice-port'));
+      return int.parse(devicePort);
     } on FormatException catch (error) {
       throwToolExit('Invalid port for `--device-vmservice-port`: $error');
     }
@@ -504,7 +506,6 @@ abstract class FlutterCommand extends Command<void> {
 
   void addPublishPort({ bool enabledByDefault = true, bool verboseHelp = false }) {
     argParser.addFlag('publish-port',
-      negatable: true,
       hide: !verboseHelp,
       help: 'Publish the VM service port over mDNS. Disable to prevent the '
             'local network permission app dialog in debug and profile build modes (iOS devices only).',
@@ -514,7 +515,7 @@ abstract class FlutterCommand extends Command<void> {
 
   bool get disablePortPublication => !boolArg('publish-port');
 
-  void usesIpv6Flag({@required bool verboseHelp}) {
+  void usesIpv6Flag({required bool verboseHelp}) {
     argParser.addFlag(ipv6Flag,
       negatable: false,
       help: 'Binds to IPv6 localhost instead of IPv4 when the flutter tool '
@@ -525,7 +526,7 @@ abstract class FlutterCommand extends Command<void> {
     _usesIpv6Flag = true;
   }
 
-  bool get ipv6 => _usesIpv6Flag ? boolArg('ipv6') : null;
+  bool? get ipv6 => _usesIpv6Flag ? boolArg('ipv6') : null;
 
   void usesBuildNumberOption() {
     argParser.addOption('build-number',
@@ -592,22 +593,20 @@ abstract class FlutterCommand extends Command<void> {
   /// Whether this command should report null safety analytics.
   bool get reportNullSafety => false;
 
-  Duration get deviceDiscoveryTimeout {
-    if (_deviceDiscoveryTimeout == null
-        && argResults.options.contains(FlutterOptions.kDeviceTimeout)
-        && argResults.wasParsed(FlutterOptions.kDeviceTimeout)) {
-      final int timeoutSeconds = int.tryParse(stringArg(FlutterOptions.kDeviceTimeout));
+  late final Duration? deviceDiscoveryTimeout = () {
+    if (argResults?.options.contains(FlutterOptions.kDeviceTimeout) == true
+        && argResults?.wasParsed(FlutterOptions.kDeviceTimeout) == true) {
+      final int? timeoutSeconds = int.tryParse(stringArg(FlutterOptions.kDeviceTimeout)!);
       if (timeoutSeconds == null) {
         throwToolExit( 'Could not parse "--${FlutterOptions.kDeviceTimeout}" argument. It must be an integer.');
       }
-      _deviceDiscoveryTimeout = Duration(seconds: timeoutSeconds);
+      return Duration(seconds: timeoutSeconds);
     }
-    return _deviceDiscoveryTimeout;
-  }
-  Duration _deviceDiscoveryTimeout;
+    return null;
+  }();
 
   void addBuildModeFlags({
-    @required bool verboseHelp,
+    required bool verboseHelp,
     bool defaultToRelease = true,
     bool excludeDebug = false,
     bool excludeRelease = false,
@@ -669,7 +668,7 @@ abstract class FlutterCommand extends Command<void> {
     );
   }
 
-  void addBundleSkSLPathOption({ @required bool hide }) {
+  void addBundleSkSLPathOption({ required bool hide }) {
     argParser.addOption(FlutterOptions.kBundleSkSLPathOption,
       help: 'A path to a file containing precompiled SkSL shaders generated '
         'during "flutter run". These can be included in an application to '
@@ -680,26 +679,24 @@ abstract class FlutterCommand extends Command<void> {
   }
 
   void addTreeShakeIconsFlag({
-    bool enabledByDefault
+    bool? enabledByDefault
   }) {
     argParser.addFlag('tree-shake-icons',
-      negatable: true,
       defaultsTo: enabledByDefault
         ?? kIconTreeShakerEnabledDefault,
       help: 'Tree shake icon fonts so that only glyphs used by the application remain.',
     );
   }
 
-  void addShrinkingFlag({ @required bool verboseHelp }) {
+  void addShrinkingFlag({ required bool verboseHelp }) {
     argParser.addFlag('shrink',
-      negatable: true,
       hide: !verboseHelp,
       help: 'This flag has no effect. Code shrinking is always enabled in release builds. '
             'To learn more, see: https://developer.android.com/studio/build/shrink-code'
     );
   }
 
-  void addNullSafetyModeOptions({ @required bool hide }) {
+  void addNullSafetyModeOptions({ required bool hide }) {
     argParser.addFlag(FlutterOptions.kNullSafety,
       help:
         'Whether to override the inferred null safety mode. This allows null-safe '
@@ -720,13 +717,12 @@ abstract class FlutterCommand extends Command<void> {
 
   /// Enables support for the hidden options --extra-front-end-options and
   /// --extra-gen-snapshot-options.
-  void usesExtraDartFlagOptions({ @required bool verboseHelp }) {
+  void usesExtraDartFlagOptions({ required bool verboseHelp }) {
     argParser.addMultiOption(FlutterOptions.kExtraFrontEndOptions,
       aliases: <String>[ kExtraFrontEndOptions ], // supported for historical reasons
       help: 'A comma-separated list of additional command line arguments that will be passed directly to the Dart front end. '
             'For example, "--${FlutterOptions.kExtraFrontEndOptions}=--enable-experiment=nonfunction-type-aliases".',
       valueHelp: '--foo,--bar',
-      splitCommas: true,
       hide: !verboseHelp,
     );
     argParser.addMultiOption(FlutterOptions.kExtraGenSnapshotOptions,
@@ -735,7 +731,6 @@ abstract class FlutterCommand extends Command<void> {
             '(Only used in "--profile" or "--release" builds.) '
             'For example, "--${FlutterOptions.kExtraGenSnapshotOptions}=--no-strip".',
       valueHelp: '--foo,--bar',
-      splitCommas: true,
       hide: !verboseHelp,
     );
   }
@@ -757,7 +752,7 @@ abstract class FlutterCommand extends Command<void> {
     );
   }
 
-  void addEnableExperimentation({ @required bool hide }) {
+  void addEnableExperimentation({ required bool hide }) {
     argParser.addMultiOption(
       FlutterOptions.kEnableExperiment,
       help:
@@ -810,7 +805,7 @@ abstract class FlutterCommand extends Command<void> {
     );
   }
 
-  void usesInitializeFromDillOption({ @required bool hide }) {
+  void usesInitializeFromDillOption({ required bool hide }) {
     argParser.addOption(FlutterOptions.kInitializeFromDill,
       help: 'Initializes the resident compiler with a specific kernel file instead of '
         'the default cached location.',
@@ -820,7 +815,6 @@ abstract class FlutterCommand extends Command<void> {
 
   void addMultidexOption({ bool hide = false }) {
     argParser.addFlag('multidex',
-      negatable: true,
       defaultsTo: true,
       help: 'When enabled, indicates that the app should be built with multidex support. This '
             'flag adds the dependencies for multidex when the minimum android sdk is 20 or '
@@ -829,7 +823,7 @@ abstract class FlutterCommand extends Command<void> {
   }
 
   /// Adds build options common to all of the desktop build commands.
-  void addCommonDesktopBuildOptions({ @required bool verboseHelp }) {
+  void addCommonDesktopBuildOptions({ required bool verboseHelp }) {
     addBuildModeFlags(verboseHelp: verboseHelp);
     addBuildPerformanceFile(hide: !verboseHelp);
     addBundleSkSLPathOption(hide: !verboseHelp);
@@ -850,7 +844,7 @@ abstract class FlutterCommand extends Command<void> {
   /// explicitly specified.
   ///
   /// Use [getBuildMode] to obtain the actual effective build mode.
-  BuildMode defaultBuildMode;
+  BuildMode defaultBuildMode = BuildMode.debug;
 
   BuildMode getBuildMode() {
     // No debug when _excludeDebug is true.
@@ -866,7 +860,7 @@ abstract class FlutterCommand extends Command<void> {
     ];
     if (modeFlags.where((bool flag) => flag).length > 1) {
       throw UsageException('Only one of "--debug", "--profile", "--jit-release", '
-                           'or "--release" can be specified.', null);
+                           'or "--release" can be specified.', '');
     }
     if (debugResult) {
       return BuildMode.debug;
@@ -892,7 +886,7 @@ abstract class FlutterCommand extends Command<void> {
     );
   }
 
-  void usesTrackWidgetCreation({ bool hasEffect = true, @required bool verboseHelp }) {
+  void usesTrackWidgetCreation({ bool hasEffect = true, required bool verboseHelp }) {
     argParser.addFlag(
       'track-widget-creation',
       hide: !hasEffect && !verboseHelp,
@@ -905,7 +899,6 @@ abstract class FlutterCommand extends Command<void> {
   void usesAnalyzeSizeFlag() {
     argParser.addFlag(
       FlutterOptions.kAnalyzeSize,
-      defaultsTo: false,
       help: 'Whether to produce additional profile information for artifact output size. '
             'This flag is only supported on "--release" builds. When building for Android, a single '
             'ABI must be specified at a time with the "--target-platform" flag. When building for iOS, '
@@ -928,11 +921,11 @@ abstract class FlutterCommand extends Command<void> {
   ///
   /// Throws a [ToolExit] if the current set of options is not compatible with
   /// each other.
-  Future<BuildInfo> getBuildInfo({ BuildMode forcedBuildMode, File forcedTargetFile }) async {
+  Future<BuildInfo> getBuildInfo({ BuildMode? forcedBuildMode, File? forcedTargetFile }) async {
     final bool trackWidgetCreation = argParser.options.containsKey('track-widget-creation') &&
       boolArg('track-widget-creation');
 
-    final String buildNumber = argParser.options.containsKey('build-number')
+    final String? buildNumber = argParser.options.containsKey('build-number')
       ? stringArg('build-number')
       : null;
 
@@ -962,7 +955,7 @@ abstract class FlutterCommand extends Command<void> {
       }
     }
 
-    String codeSizeDirectory;
+    String? codeSizeDirectory;
     if (argParser.options.containsKey(FlutterOptions.kAnalyzeSize) && boolArg(FlutterOptions.kAnalyzeSize)) {
       Directory directory = globals.fsUtils.getUniqueDirectory(
         globals.fs.directory(getBuildDirectory()),
@@ -980,13 +973,13 @@ abstract class FlutterCommand extends Command<void> {
       // Explicitly check for `true` and `false` so that `null` results in not
       // passing a flag. Examine the entrypoint file to determine if it
       // is opted in or out.
-      final bool wasNullSafetyFlagParsed = argResults.wasParsed(FlutterOptions.kNullSafety);
+      final bool wasNullSafetyFlagParsed = argResults?.wasParsed(FlutterOptions.kNullSafety) == true;
       if (!wasNullSafetyFlagParsed && (argParser.options.containsKey('target') || forcedTargetFile != null)) {
         final File entrypointFile = forcedTargetFile ?? globals.fs.file(targetFile);
         final LanguageVersion languageVersion = determineLanguageVersion(
           entrypointFile,
           packageConfig.packageOf(entrypointFile.absolute.uri),
-          Cache.flutterRoot,
+          Cache.flutterRoot!,
         );
         // Extra frontend options are only provided if explicitly
         // requested.
@@ -1011,7 +1004,7 @@ abstract class FlutterCommand extends Command<void> {
     final bool dartObfuscation = argParser.options.containsKey(FlutterOptions.kDartObfuscationOption)
       && boolArg(FlutterOptions.kDartObfuscationOption);
 
-    final String splitDebugInfoPath = argParser.options.containsKey(FlutterOptions.kSplitDebugInfoOption)
+    final String? splitDebugInfoPath = argParser.options.containsKey(FlutterOptions.kSplitDebugInfoOption)
       ? stringArg(FlutterOptions.kSplitDebugInfoOption)
       : null;
 
@@ -1037,10 +1030,10 @@ abstract class FlutterCommand extends Command<void> {
     }
 
     final bool treeShakeIcons = argParser.options.containsKey('tree-shake-icons')
-      && buildMode.isPrecompiled
+      && buildMode.isPrecompiled == true
       && boolArg('tree-shake-icons');
 
-    final String bundleSkSLPath = argParser.options.containsKey(FlutterOptions.kBundleSkSLPathOption)
+    final String? bundleSkSLPath = argParser.options.containsKey(FlutterOptions.kBundleSkSLPathOption)
       ? stringArg(FlutterOptions.kBundleSkSLPathOption)
       : null;
 
@@ -1048,7 +1041,7 @@ abstract class FlutterCommand extends Command<void> {
       throwToolExit('No SkSL shader bundle found at $bundleSkSLPath.');
     }
 
-    final String performanceMeasurementFile = argParser.options.containsKey(FlutterOptions.kPerformanceMeasurementFile)
+    final String? performanceMeasurementFile = argParser.options.containsKey(FlutterOptions.kPerformanceMeasurementFile)
       ? stringArg(FlutterOptions.kPerformanceMeasurementFile)
       : null;
 
@@ -1057,7 +1050,7 @@ abstract class FlutterCommand extends Command<void> {
         : <String>[];
 
     if (argParser.options.containsKey('web-renderer')) {
-      dartDefines = updateDartDefines(dartDefines, stringArg('web-renderer'));
+      dartDefines = updateDartDefines(dartDefines, stringArg('web-renderer')!);
     }
 
     return BuildInfo(buildMode,
@@ -1065,10 +1058,10 @@ abstract class FlutterCommand extends Command<void> {
         ? stringArg('flavor')
         : null,
       trackWidgetCreation: trackWidgetCreation,
-      extraFrontEndOptions: extraFrontEndOptions?.isNotEmpty ?? false
+      extraFrontEndOptions: extraFrontEndOptions.isNotEmpty
         ? extraFrontEndOptions
         : null,
-      extraGenSnapshotOptions: extraGenSnapshotOptions?.isNotEmpty ?? false
+      extraGenSnapshotOptions: extraGenSnapshotOptions.isNotEmpty
         ? extraGenSnapshotOptions
         : null,
       fileSystemRoots: fileSystemRoots,
@@ -1102,10 +1095,10 @@ abstract class FlutterCommand extends Command<void> {
 
   /// The path to send to Google Analytics. Return null here to disable
   /// tracking of the command.
-  Future<String> get usagePath async {
+  Future<String?> get usagePath async {
     if (parent is FlutterCommand) {
-      final FlutterCommand commandParent = parent as FlutterCommand;
-      final String path = await commandParent.usagePath;
+      final FlutterCommand? commandParent = parent as FlutterCommand?;
+      final String? path = await commandParent?.usagePath;
       // Don't report for parents that return null for usagePath.
       return path == null ? null : '$path/$name';
     } else {
@@ -1133,15 +1126,19 @@ abstract class FlutterCommand extends Command<void> {
         // Prints the welcome message if needed.
         globals.flutterUsage.printWelcome();
         _printDeprecationWarning();
-        final String commandPath = await usagePath;
-        _registerSignalHandlers(commandPath, startTime);
+        final String? commandPath = await usagePath;
+        if (commandPath != null) {
+          _registerSignalHandlers(commandPath, startTime);
+        }
         FlutterCommandResult commandResult = FlutterCommandResult.fail();
         try {
           commandResult = await verifyThenRunCommand(commandPath);
         } finally {
           final DateTime endTime = globals.systemClock.now();
           globals.printTrace(userMessages.flutterElapsedTime(name, getElapsedAsMilliseconds(endTime.difference(startTime))));
-          _sendPostUsage(commandPath, commandResult, startTime, endTime);
+          if (commandPath != null) {
+            _sendPostUsage(commandPath, commandResult, startTime, endTime);
+          }
         }
       },
     );
@@ -1167,7 +1164,10 @@ abstract class FlutterCommand extends Command<void> {
         && dartDefines.any((String d) => d.startsWith('FLUTTER_WEB_USE_SKIA='))) {
       dartDefinesSet.removeWhere((String d) => d.startsWith('FLUTTER_WEB_USE_SKIA='));
     }
-    dartDefinesSet.addAll(_webRendererDartDefines[webRenderer]);
+    final Iterable<String>? webRendererDefine = _webRendererDartDefines[webRenderer];
+    if (webRendererDefine != null) {
+      dartDefinesSet.addAll(webRendererDefine);
+    }
     return dartDefinesSet.toList();
   }
 
@@ -1207,7 +1207,7 @@ abstract class FlutterCommand extends Command<void> {
       if (commandResult.exitStatus != null)
         getEnumName(commandResult.exitStatus),
       if (commandResult.timingLabelParts?.isNotEmpty ?? false)
-        ...commandResult.timingLabelParts,
+        ...?commandResult.timingLabelParts,
     ];
 
     final String label = labels
@@ -1233,7 +1233,7 @@ abstract class FlutterCommand extends Command<void> {
   /// then call this method to execute the command
   /// rather than calling [runCommand] directly.
   @mustCallSuper
-  Future<FlutterCommandResult> verifyThenRunCommand(String commandPath) async {
+  Future<FlutterCommandResult> verifyThenRunCommand(String? commandPath) async {
     // Populate the cache. We call this before pub get below so that the
     // sky_engine package is available in the flutter cache for pub to find.
     if (shouldUpdateCache) {
@@ -1249,7 +1249,7 @@ abstract class FlutterCommand extends Command<void> {
     if (shouldRunPub) {
       final FlutterProject project = FlutterProject.current();
       final Environment environment = Environment(
-        artifacts: globals.artifacts,
+        artifacts: globals.artifacts!,
         logger: globals.logger,
         cacheDir: globals.cache.getRoot(),
         engineVersion: globals.flutterVersion.engineRevision,
@@ -1264,7 +1264,7 @@ abstract class FlutterCommand extends Command<void> {
 
       await generateLocalizationsSyntheticPackage(
         environment: environment,
-        buildSystem: globals.buildSystem,
+        buildSystem: globals.buildSystem!,
       );
 
       await pub.get(
@@ -1314,21 +1314,21 @@ abstract class FlutterCommand extends Command<void> {
   /// devices and criteria entered by the user on the command line.
   /// If no device can be found that meets specified criteria,
   /// then print an error message and return null.
-  Future<List<Device>> findAllTargetDevices({
+  Future<List<Device>?> findAllTargetDevices({
     bool includeUnsupportedDevices = false,
   }) async {
-    if (!globals.doctor.canLaunchAnything) {
+    if (!globals.doctor!.canLaunchAnything) {
       globals.printError(userMessages.flutterNoDevelopmentDevice);
       return null;
     }
-    final DeviceManager deviceManager = globals.deviceManager;
+    final DeviceManager deviceManager = globals.deviceManager!;
     List<Device> devices = await deviceManager.findTargetDevices(
       includeUnsupportedDevices ? null : FlutterProject.current(),
       timeout: deviceDiscoveryTimeout,
     );
 
     if (devices.isEmpty && deviceManager.hasSpecifiedDeviceId) {
-      globals.printStatus(userMessages.flutterNoMatchingDevice(deviceManager.specifiedDeviceId));
+      globals.printStatus(userMessages.flutterNoMatchingDevice(deviceManager.specifiedDeviceId!));
       return null;
     } else if (devices.isEmpty) {
       if (deviceManager.hasSpecifiedAllDevices) {
@@ -1346,7 +1346,7 @@ abstract class FlutterCommand extends Command<void> {
               .toList(),
           '\n',
         );
-        result.writeln('');
+        result.writeln();
         result.writeln(userMessages.flutterMissPlatformProjects(
           Device.devicesPlatformTypes(unsupportedDevices),
         ));
@@ -1355,7 +1355,7 @@ abstract class FlutterCommand extends Command<void> {
       return null;
     } else if (devices.length > 1 && !deviceManager.hasSpecifiedAllDevices) {
       if (deviceManager.hasSpecifiedDeviceId) {
-       globals.printStatus(userMessages.flutterFoundSpecifiedDevices(devices.length, deviceManager.specifiedDeviceId));
+       globals.printStatus(userMessages.flutterFoundSpecifiedDevices(devices.length, deviceManager.specifiedDeviceId!));
       } else {
         globals.printStatus(userMessages.flutterSpecifyDeviceWithAllOption);
         devices = await deviceManager.getAllConnectedDevices();
@@ -1374,16 +1374,16 @@ abstract class FlutterCommand extends Command<void> {
   ///
   /// If [includeUnsupportedDevices] is true, the tool does not filter
   /// the list by the current project support list.
-  Future<Device> findTargetDevice({
+  Future<Device?> findTargetDevice({
     bool includeUnsupportedDevices = false,
   }) async {
-    List<Device> deviceList = await findAllTargetDevices(includeUnsupportedDevices: includeUnsupportedDevices);
+    List<Device>? deviceList = await findAllTargetDevices(includeUnsupportedDevices: includeUnsupportedDevices);
     if (deviceList == null) {
       return null;
     }
     if (deviceList.length > 1) {
       globals.printStatus(userMessages.flutterSpecifyDevice);
-      deviceList = await globals.deviceManager.getAllConnectedDevices();
+      deviceList = await globals.deviceManager!.getAllConnectedDevices();
       globals.printStatus('');
       await Device.printDevices(deviceList, globals.logger);
       return null;
@@ -1394,7 +1394,7 @@ abstract class FlutterCommand extends Command<void> {
   @protected
   @mustCallSuper
   Future<void> validateCommand() async {
-    if (_requiresPubspecYaml && !globalResults.wasParsed('packages')) {
+    if (_requiresPubspecYaml && globalResults?.wasParsed('packages') != true) {
       // Don't expect a pubspec.yaml file if the user passed in an explicit .packages file path.
 
       // If there is no pubspec in the current directory, look in the parent
@@ -1433,23 +1433,23 @@ abstract class FlutterCommand extends Command<void> {
       description,
       '',
       'Global options:',
-      runner.argParser.usage,
+      '${runner?.argParser.usage}',
       '',
       usageWithoutDescription,
     ].join('\n');
     return help;
   }
 
-  ApplicationPackageFactory applicationPackages;
+  ApplicationPackageFactory? applicationPackages;
 
   /// Gets the parsed command-line option named [name] as `bool`.
-  bool boolArg(String name) => argResults[name] as bool;
+  bool boolArg(String name) => argResults?[name] as bool? ?? false;
 
   /// Gets the parsed command-line option named [name] as `String`.
-  String stringArg(String name) => argResults[name] as String;
+  String? stringArg(String name) => argResults?[name] as String?;
 
   /// Gets the parsed command-line option named [name] as `List<String>`.
-  List<String> stringsArg(String name) => argResults[name] as List<String>;
+  List<String> stringsArg(String name) => argResults?[name] as List<String>? ?? <String>[];
 }
 
 /// A mixin which applies an implementation of [requiredArtifacts] that only
@@ -1460,7 +1460,7 @@ mixin DeviceBasedDevelopmentArtifacts on FlutterCommand {
     // If there are no attached devices, use the default configuration.
     // Otherwise, only add development artifacts which correspond to a
     // connected device.
-    final List<Device> devices = await globals.deviceManager.getDevices();
+    final List<Device> devices = await globals.deviceManager!.getDevices();
     if (devices.isEmpty) {
       return super.requiredArtifacts;
     }
@@ -1469,7 +1469,7 @@ mixin DeviceBasedDevelopmentArtifacts on FlutterCommand {
     };
     for (final Device device in devices) {
       final TargetPlatform targetPlatform = await device.targetPlatform;
-      final DevelopmentArtifact developmentArtifact = artifactFromTargetPlatform(targetPlatform);
+      final DevelopmentArtifact? developmentArtifact = artifactFromTargetPlatform(targetPlatform);
       if (developmentArtifact != null) {
         artifacts.add(developmentArtifact);
       }
@@ -1481,7 +1481,7 @@ mixin DeviceBasedDevelopmentArtifacts on FlutterCommand {
 // Returns the development artifact for the target platform, or null
 // if none is supported
 @protected
-DevelopmentArtifact artifactFromTargetPlatform(TargetPlatform targetPlatform) {
+DevelopmentArtifact? artifactFromTargetPlatform(TargetPlatform targetPlatform) {
   switch (targetPlatform) {
     case TargetPlatform.android:
     case TargetPlatform.android_arm:
@@ -1518,7 +1518,6 @@ DevelopmentArtifact artifactFromTargetPlatform(TargetPlatform targetPlatform) {
       }
       return null;
   }
-  return null;
 }
 
 /// Returns true if s is either null, empty or is solely made of whitespace characters (as defined by String.trim).
diff --git a/packages/flutter_tools/lib/src/runner/flutter_command_runner.dart b/packages/flutter_tools/lib/src/runner/flutter_command_runner.dart
index 4030bbe757..396756808e 100644
--- a/packages/flutter_tools/lib/src/runner/flutter_command_runner.dart
+++ b/packages/flutter_tools/lib/src/runner/flutter_command_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 'package:args/args.dart';
 import 'package:args/command_runner.dart';
 import 'package:completion/completion.dart';
@@ -45,14 +43,12 @@ class FlutterCommandRunner extends CommandRunner<void> {
     argParser.addFlag('prefixed-errors',
         negatable: false,
         help: 'Causes lines sent to stderr to be prefixed with "ERROR:".',
-        hide: !verboseHelp,
-        defaultsTo: false);
+        hide: !verboseHelp);
     argParser.addFlag('quiet',
         negatable: false,
         hide: !verboseHelp,
         help: 'Reduce the amount of output from some commands.');
     argParser.addFlag('wrap',
-        negatable: true,
         hide: !verboseHelp,
         help: 'Toggles output word wrapping, regardless of whether or not the output is a terminal.',
         defaultsTo: true);
@@ -60,8 +56,7 @@ class FlutterCommandRunner extends CommandRunner<void> {
         hide: !verboseHelp,
         help: 'Sets the output wrap column. If not set, uses the width of the terminal. No '
               'wrapping occurs if not writing to a terminal. Use "--no-wrap" to turn off wrapping '
-              'when connected to a terminal.',
-        defaultsTo: null);
+              'when connected to a terminal.');
     argParser.addOption('device-id',
         abbr: 'd',
         help: 'Target device id or name (prefixes allowed).');
@@ -73,12 +68,10 @@ class FlutterCommandRunner extends CommandRunner<void> {
         hide: !verboseHelp,
         help: 'When used with the "--version" flag, outputs the information using JSON.');
     argParser.addFlag('color',
-        negatable: true,
         hide: !verboseHelp,
         help: 'Whether to use terminal colors (requires support for ANSI escape sequences).',
         defaultsTo: true);
     argParser.addFlag('version-check',
-        negatable: true,
         defaultsTo: true,
         hide: !verboseHelp,
         help: 'Allow Flutter to check for updates when this command runs.');
@@ -158,12 +151,12 @@ class FlutterCommandRunner extends CommandRunner<void> {
         usageException(error.message);
       }
 
-      Command<void> command = commands[error.commands.first];
+      Command<void>? command = commands[error.commands.first];
       for (final String commandName in error.commands.skip(1)) {
-        command = command.subcommands[commandName];
+        command = command?.subcommands[commandName];
       }
 
-      command.usageException(error.message);
+      command!.usageException(error.message);
     }
   }
 
@@ -184,12 +177,12 @@ class FlutterCommandRunner extends CommandRunner<void> {
 
   @override
   Future<void> runCommand(ArgResults topLevelResults) async {
-    final Map<Type, dynamic> contextOverrides = <Type, dynamic>{};
+    final Map<Type, Object?> contextOverrides = <Type, Object?>{};
 
     // Don't set wrapColumns unless the user said to: if it's set, then all
     // wrapping will occur at this width explicitly, and won't adapt if the
     // terminal size changes during a run.
-    int wrapColumn;
+    int? wrapColumn;
     if (topLevelResults.wasParsed('wrap-column')) {
       try {
         wrapColumn = int.parse(topLevelResults['wrap-column'] as String);
@@ -208,53 +201,53 @@ class FlutterCommandRunner extends CommandRunner<void> {
         : globals.stdio.terminalColumns != null && topLevelResults['wrap'] as bool;
     contextOverrides[OutputPreferences] = OutputPreferences(
       wrapText: useWrapping,
-      showColor: topLevelResults['color'] as bool,
+      showColor: topLevelResults['color'] as bool?,
       wrapColumn: wrapColumn,
     );
 
-    if (topLevelResults['show-test-device'] as bool ||
+    if ((topLevelResults['show-test-device'] as bool?) == true ||
         topLevelResults['device-id'] == FlutterTesterDevices.kTesterDeviceId) {
       FlutterTesterDevices.showFlutterTesterDevice = true;
     }
-    if (topLevelResults['show-web-server-device'] as bool ||
+    if ((topLevelResults['show-web-server-device'] as bool?) == true  ||
         topLevelResults['device-id'] == WebServerDevice.kWebServerDeviceId) {
       WebServerDevice.showWebServerDevice = true;
     }
 
     // Set up the tooling configuration.
-    final EngineBuildPaths engineBuildPaths = await globals.localEngineLocator.findEnginePath(
-      topLevelResults['local-engine-src-path'] as String,
-      topLevelResults['local-engine'] as String,
-      topLevelResults['packages'] as String,
+    final EngineBuildPaths? engineBuildPaths = await globals.localEngineLocator?.findEnginePath(
+      topLevelResults['local-engine-src-path'] as String?,
+      topLevelResults['local-engine'] as String?,
+      topLevelResults['packages'] as String?,
     );
     if (engineBuildPaths != null) {
-      contextOverrides.addAll(<Type, dynamic>{
+      contextOverrides.addAll(<Type, Object?>{
         Artifacts: Artifacts.getLocalEngine(engineBuildPaths),
       });
     }
 
     await context.run<void>(
-      overrides: contextOverrides.map<Type, Generator>((Type type, dynamic value) {
+      overrides: contextOverrides.map<Type, Generator>((Type type, Object? value) {
         return MapEntry<Type, Generator>(type, () => value);
       }),
       body: () async {
-        globals.logger.quiet = topLevelResults['quiet'] as bool;
+        globals.logger.quiet = (topLevelResults['quiet'] as bool?) == true;
 
         if (globals.platform.environment['FLUTTER_ALREADY_LOCKED'] != 'true') {
           await globals.cache.lock();
         }
 
-        if (topLevelResults['suppress-analytics'] as bool) {
+        if ((topLevelResults['suppress-analytics'] as bool?) == true) {
           globals.flutterUsage.suppressAnalytics = true;
         }
 
         globals.flutterVersion.ensureVersionFile();
-        final bool machineFlag = topLevelResults['machine'] as bool;
+        final bool machineFlag = topLevelResults['machine'] as bool? ?? false;
         final bool ci = await globals.botDetector.isRunningOnBot;
         final bool redirectedCompletion = !globals.stdio.hasTerminal &&
             (topLevelResults.command?.name ?? '').endsWith('-completion');
         final bool isMachine = machineFlag || ci || redirectedCompletion;
-        final bool versionCheckFlag = topLevelResults['version-check'] as bool;
+        final bool versionCheckFlag = topLevelResults['version-check'] as bool? ?? false;
         final bool explicitVersionCheckPassed = topLevelResults.wasParsed('version-check') && versionCheckFlag;
 
         if (topLevelResults.command?.name != 'upgrade' &&
@@ -263,16 +256,16 @@ class FlutterCommandRunner extends CommandRunner<void> {
         }
 
         // See if the user specified a specific device.
-        globals.deviceManager.specifiedDeviceId = topLevelResults['device-id'] as String;
+        globals.deviceManager?.specifiedDeviceId = topLevelResults['device-id'] as String?;
 
-        if (topLevelResults['version'] as bool) {
+        if ((topLevelResults['version'] as bool?) == true) {
           globals.flutterUsage.sendCommand('version');
           globals.flutterVersion.fetchTagsAndUpdate();
           String status;
           if (machineFlag) {
             final Map<String, Object> jsonOut = globals.flutterVersion.toJson();
             if (jsonOut != null) {
-              jsonOut['flutterRoot'] = Cache.flutterRoot;
+              jsonOut['flutterRoot'] = Cache.flutterRoot!;
             }
             status = const JsonEncoder.withIndent('  ').convert(jsonOut);
           } else {
@@ -292,7 +285,7 @@ class FlutterCommandRunner extends CommandRunner<void> {
 
   /// Get the root directories of the repo - the directories containing Dart packages.
   List<String> getRepoRoots() {
-    final String root = globals.fs.path.absolute(Cache.flutterRoot);
+    final String root = globals.fs.path.absolute(Cache.flutterRoot!);
     // not bin, and not the root
     return <String>['dev', 'examples', 'packages'].map<String>((String item) {
       return globals.fs.path.join(root, item);