migrate foundation to nullsafety (#61188)

* migrate foundation to nullsafety

* address review comments

packages/flutter/analysis_options.yaml

@@ -5,3 +5,7 @@ include: ../analysis_options.yaml
 analyzer:
   enable-experiment:
     - non-nullable
+  errors:
+    always_require_non_null_named_parameters: false # not needed with nnbd
+    void_checks: false # https://github.com/dart-lang/linter/issues/2185
+    unnecessary_null_comparison: false # https://github.com/dart-lang/language/issues/1018 , turned off until https://github.com/flutter/flutter/issues/61042

packages/flutter/lib/foundation.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
-
 /// Core Flutter framework primitives.
 ///
 /// The features defined in this library are the lowest-level utility

packages/flutter/lib/src/foundation/_bitfield_io.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 'bitfield.dart' as bitfield;
 
 /// The dart:io implementation of [bitfield.kMaxUnsignedSMI].

packages/flutter/lib/src/foundation/_bitfield_web.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 'bitfield.dart' as bitfield;
 
 /// The dart:html implementation of [bitfield.kMaxUnsignedSMI].

packages/flutter/lib/src/foundation/_isolates_io.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:developer';
 import 'dart:isolate';
@@ -13,7 +11,7 @@ import 'constants.dart';
 import 'isolates.dart' as isolates;
 
 /// The dart:io implementation of [isolate.compute].
-Future<R> compute<Q, R>(isolates.ComputeCallback<Q, R> callback, Q message, { String debugLabel }) async {
+Future<R> compute<Q, R>(isolates.ComputeCallback<Q, R> callback, Q message, { String? debugLabel }) async {
   debugLabel ??= kReleaseMode ? 'compute' : callback.toString();
   final Flow flow = Flow.begin();
   Timeline.startSync('$debugLabel: start', flow: flow);

packages/flutter/lib/src/foundation/_isolates_web.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 'isolates.dart' as isolates;
 
 /// The dart:html implementation of [isolate.compute].
-Future<R> compute<Q, R>(isolates.ComputeCallback<Q, R> callback, Q message, { String debugLabel }) async {
+Future<R> compute<Q, R>(isolates.ComputeCallback<Q, R> callback, Q message, { String? debugLabel }) async {
   // To avoid blocking the UI immediately for an expensive function call, we
   // pump a single frame to allow the framework to complete the current set
   // of work.

packages/flutter/lib/src/foundation/_platform_io.dart

@@ -2,15 +2,13 @@
 // 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 'assertions.dart';
 import 'platform.dart' as platform;
 
 /// The dart:io implementation of [platform.defaultTargetPlatform].
 platform.TargetPlatform get defaultTargetPlatform {
-  platform.TargetPlatform result;
+  platform.TargetPlatform? result;
   if (Platform.isAndroid) {
     result = platform.TargetPlatform.android;
   } else if (Platform.isIOS) {
@@ -38,5 +36,5 @@ platform.TargetPlatform get defaultTargetPlatform {
       'Consider updating the list of TargetPlatforms to include this platform.'
     );
   }
-  return result;
+  return result!;
 }

packages/flutter/lib/src/foundation/_platform_web.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:html' as html;
 import 'platform.dart' as platform;
 
@@ -14,7 +12,7 @@ platform.TargetPlatform get defaultTargetPlatform {
   // platforms configuration for Flutter.
   platform.TargetPlatform result = _browserPlatform();
   if (platform.debugDefaultTargetPlatformOverride != null)
-    result = platform.debugDefaultTargetPlatformOverride;
+    result = platform.debugDefaultTargetPlatformOverride!;
   return result;
 }
 

packages/flutter/lib/src/foundation/annotations.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
-
 // Examples can assume:
 // class Cat { }
 

packages/flutter/lib/src/foundation/assertions.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:meta/meta.dart';
 
 import 'basic_types.dart';
@@ -46,9 +44,9 @@ class PartialStackFrame {
   /// Creates a new [PartialStackFrame] instance. All arguments are required and
   /// must not be null.
   const PartialStackFrame({
-    @required this.package,
-    @required this.className,
-    @required this.method,
+    required this.package,
+    required this.className,
+    required this.method,
   }) : assert(className != null),
        assert(method != null),
        assert(package != null);
@@ -102,7 +100,7 @@ abstract class StackFilter {
   /// `reasons`.
   ///
   /// To elide a frame or number of frames, set the string
-  void filter(List<StackFrame> stackFrames, List<String> reasons);
+  void filter(List<StackFrame> stackFrames, List<String?> reasons);
 }
 
 
@@ -120,8 +118,8 @@ class RepetitiveStackFrameFilter extends StackFilter {
   /// Creates a new RepetitiveStackFrameFilter. All parameters are required and must not be
   /// null.
   const RepetitiveStackFrameFilter({
-    @required this.frames,
-    @required this.replacement,
+    required this.frames,
+    required this.replacement,
   }) : assert(frames != null),
        assert(replacement != null);
 
@@ -141,7 +139,7 @@ class RepetitiveStackFrameFilter extends StackFilter {
   List<String> get _replacements => List<String>.filled(numFrames, replacement);
 
   @override
-  void filter(List<StackFrame> stackFrames, List<String> reasons) {
+  void filter(List<StackFrame> stackFrames, List<String?> reasons) {
     for (int index = 0; index < stackFrames.length - numFrames; index += 1) {
       if (_matchesFrames(stackFrames.skip(index).take(numFrames).toList())) {
         reasons.setRange(index, index + numFrames, _replacements);
@@ -222,8 +220,12 @@ abstract class _ErrorDiagnostic extends DiagnosticsProperty<List<Object>> {
          level: level,
        );
 
+
+  @override
+  List<Object> get value => super.value!;
+
   @override
-  String valueToString({ TextTreeConfiguration parentConfiguration }) {
+  String valueToString({ TextTreeConfiguration? parentConfiguration }) {
     return value.join('');
   }
 }
@@ -400,13 +402,13 @@ class FlutterErrorDetails with Diagnosticable {
   /// Creates a copy of the error details but with the given fields replaced
   /// with new values.
   FlutterErrorDetails copyWith({
-    DiagnosticsNode context,
+    DiagnosticsNode? context,
     dynamic exception,
-    InformationCollector informationCollector,
-    String library,
-    bool silent,
-    StackTrace stack,
-    IterableFilter<String> stackFilter,
+    InformationCollector? informationCollector,
+    String? library,
+    bool? silent,
+    StackTrace? stack,
+    IterableFilter<String>? stackFilter,
   }) {
     return FlutterErrorDetails(
       context: context ?? this.context,
@@ -448,12 +450,12 @@ class FlutterErrorDetails with Diagnosticable {
   /// callback, then [FlutterError.defaultStackFilter] is used instead. That
   /// function expects the stack to be in the format used by
   /// [StackTrace.toString].
-  final StackTrace stack;
+  final StackTrace? stack;
 
   /// A human-readable brief name describing the library that caught the error
   /// message. This is used by the default error handler in the header dumped to
   /// the console.
-  final String library;
+  final String? library;
 
   /// A [DiagnosticsNode] that provides a human-readable description of where
   /// the error was caught (as opposed to where it was thrown).
@@ -494,7 +496,7 @@ class FlutterErrorDetails with Diagnosticable {
   ///    applicable.
   ///  * [FlutterError], which is the most common place to use
   ///    [FlutterErrorDetails].
-  final DiagnosticsNode context;
+  final DiagnosticsNode? context;
 
   /// A callback which filters the [stack] trace. Receives an iterable of
   /// strings representing the frames encoded in the way that
@@ -510,7 +512,7 @@ class FlutterErrorDetails with Diagnosticable {
   /// that function, however, does not always follow this format.
   ///
   /// This won't be called if [stack] is null.
-  final IterableFilter<String> stackFilter;
+  final IterableFilter<String>? stackFilter;
 
   /// A callback which, when called with a [StringBuffer] will write to that buffer
   /// information that could help with debugging the problem.
@@ -520,7 +522,7 @@ class FlutterErrorDetails with Diagnosticable {
   ///
   /// The text written to the information argument may contain newlines but should
   /// not end with a newline.
-  final InformationCollector informationCollector;
+  final InformationCollector? informationCollector;
 
   /// Whether this error should be ignored by the default error reporting
   /// behavior in release mode.
@@ -544,13 +546,13 @@ class FlutterErrorDetails with Diagnosticable {
   /// prettier, to handle exceptions that stringify to empty strings, to handle
   /// objects that don't inherit from [Exception] or [Error], and so forth.
   String exceptionAsString() {
-    String longMessage;
+    String? longMessage;
     if (exception is AssertionError) {
       // Regular _AssertionErrors thrown by assert() put the message last, after
       // some code snippets. This leads to ugly messages. To avoid this, we move
       // the assertion message up to before the code snippets, separated by a
       // newline, if we recognize that format is being used.
-      final Object message = exception.message;
+      final Object? message = exception.message;
       final String fullMessage = exception.toString();
       if (message is String && message != fullMessage) {
         if (fullMessage.length > message.length) {
@@ -583,7 +585,7 @@ class FlutterErrorDetails with Diagnosticable {
     return longMessage;
   }
 
-  Diagnosticable _exceptionToDiagnosticable() {
+  Diagnosticable? _exceptionToDiagnosticable() {
     if (exception is FlutterError) {
       return exception as FlutterError;
     }
@@ -606,12 +608,12 @@ class FlutterErrorDetails with Diagnosticable {
     if (kReleaseMode) {
       return DiagnosticsNode.message(formatException());
     }
-    final Diagnosticable diagnosticable = _exceptionToDiagnosticable();
-    DiagnosticsNode summary;
+    final Diagnosticable? diagnosticable = _exceptionToDiagnosticable();
+    DiagnosticsNode? summary;
     if (diagnosticable != null) {
       final DiagnosticPropertiesBuilder builder = DiagnosticPropertiesBuilder();
       debugFillProperties(builder);
-      summary = builder.properties.firstWhere((DiagnosticsNode node) => node.level == DiagnosticLevel.summary, orElse: () => null);
+      summary = builder.properties.cast<DiagnosticsNode?>().firstWhere((DiagnosticsNode? node) => node!.level == DiagnosticLevel.summary, orElse: () => null);
     }
     return summary ?? ErrorSummary(formatException());
   }
@@ -620,7 +622,7 @@ class FlutterErrorDetails with Diagnosticable {
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     final DiagnosticsNode verb = ErrorDescription('thrown${ context != null ? ErrorDescription(" $context") : ""}');
-    final Diagnosticable diagnosticable = _exceptionToDiagnosticable();
+    final Diagnosticable? diagnosticable = _exceptionToDiagnosticable();
     if (exception is NullThrownError) {
       properties.add(ErrorDescription('The null value was $verb.'));
     } else if (exception is num) {
@@ -659,7 +661,7 @@ class FlutterErrorDetails with Diagnosticable {
         // If not: Error is in user code (user violated assertion in framework).
         // If so:  Error is in Framework. We either need an assertion higher up
         //         in the stack, or we've violated our own assertions.
-        final List<StackFrame> stackFrames = StackFrame.fromStackTrace(FlutterError.demangleStackTrace(stack))
+        final List<StackFrame> stackFrames = StackFrame.fromStackTrace(FlutterError.demangleStackTrace(stack!))
                                                        .skipWhile((StackFrame frame) => frame.packageScheme == 'dart')
                                                        .toList();
         final bool ourFault =  stackFrames.length >= 2
@@ -677,11 +679,11 @@ class FlutterErrorDetails with Diagnosticable {
         }
       }
       properties.add(ErrorSpacer());
-      properties.add(DiagnosticsStackTrace('When the exception was thrown, this was the stack', stack, stackFilter: stackFilter));
+      properties.add(DiagnosticsStackTrace('When the exception was thrown, this was the stack', stack!, stackFilter: stackFilter));
     }
     if (informationCollector != null) {
       properties.add(ErrorSpacer());
-      informationCollector().forEach(properties.add);
+      informationCollector!().forEach(properties.add);
     }
   }
 
@@ -696,7 +698,7 @@ class FlutterErrorDetails with Diagnosticable {
   }
 
   @override
-  DiagnosticsNode toDiagnosticsNode({ String name, DiagnosticsTreeStyle style }) {
+  DiagnosticsNode toDiagnosticsNode({ String? name, DiagnosticsTreeStyle? style }) {
     return _FlutterErrorDetailsNode(
       name: name,
       value: this,
@@ -866,9 +868,6 @@ class FlutterError extends Error with DiagnosticableTreeMixin implements Asserti
   ///
   /// If the error handler throws an exception, it will not be caught by the
   /// Flutter framework.
-  ///
-  /// Set this to null to silently catch and ignore errors. This is not
-  /// recommended.
   static FlutterExceptionHandler onError = (FlutterErrorDetails details) => presentError(details);
 
   /// Called by the Flutter framework before attempting to parse a [StackTrace].
@@ -1005,17 +1004,17 @@ class FlutterError extends Error with DiagnosticableTreeMixin implements Asserti
       final String package = '${frame.packageScheme}:${frame.package}';
       if (removedPackagesAndClasses.containsKey(className)) {
         skipped += 1;
-        removedPackagesAndClasses[className] += 1;
+        removedPackagesAndClasses.update(className, (int value) => value + 1);
         parsedFrames.removeAt(index);
         index -= 1;
       } else if (removedPackagesAndClasses.containsKey(package)) {
         skipped += 1;
-        removedPackagesAndClasses[package] += 1;
+        removedPackagesAndClasses.update(package, (int value) => value + 1);
         parsedFrames.removeAt(index);
         index -= 1;
       }
     }
-    final List<String> reasons = List<String>(parsedFrames.length);
+    final List<String?> reasons = List<String?>.filled(parsedFrames.length, null, growable: false);
     for (final StackFilter filter in _stackFilters) {
       filter.filter(parsedFrames, reasons);
     }
@@ -1062,7 +1061,7 @@ class FlutterError extends Error with DiagnosticableTreeMixin implements Asserti
 
   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
-    diagnostics?.forEach(properties.add);
+    diagnostics.forEach(properties.add);
   }
 
   @override
@@ -1079,11 +1078,10 @@ class FlutterError extends Error with DiagnosticableTreeMixin implements Asserti
     return diagnostics.map((DiagnosticsNode node) => renderer.render(node).trimRight()).join('\n');
   }
 
-  /// Calls [onError] with the given details, unless it is null.
+  /// Calls [onError] with the given details.
   static void reportError(FlutterErrorDetails details) {
     assert(details != null);
     assert(details.exception != null);
-    if (onError != null)
     onError(details);
   }
 }
@@ -1099,7 +1097,7 @@ class FlutterError extends Error with DiagnosticableTreeMixin implements Asserti
 /// included.
 ///
 /// The `label` argument, if present, will be printed before the stack.
-void debugPrintStack({StackTrace stackTrace, String label, int maxFrames}) {
+void debugPrintStack({StackTrace? stackTrace, String? label, int? maxFrames}) {
   if (label != null)
     debugPrint(label);
   if (stackTrace == null) {
@@ -1136,8 +1134,8 @@ class DiagnosticsStackTrace extends DiagnosticsBlock {
   /// [showSeparator] indicates whether to include a ':' after the [name].
   DiagnosticsStackTrace(
     String name,
-    StackTrace stack, {
-    IterableFilter<String> stackFilter,
+    StackTrace? stack, {
+    IterableFilter<String>? stackFilter,
     bool showSeparator = true,
   }) : super(
     name: name,
@@ -1151,7 +1149,7 @@ class DiagnosticsStackTrace extends DiagnosticsBlock {
   /// Creates a diagnostic describing a single frame from a StackTrace.
   DiagnosticsStackTrace.singleFrame(
     String name, {
-    @required String frame,
+    required String frame,
     bool showSeparator = true,
   }) : super(
     name: name,
@@ -1161,8 +1159,8 @@ class DiagnosticsStackTrace extends DiagnosticsBlock {
   );
 
   static List<DiagnosticsNode> _applyStackFilter(
-    StackTrace stack,
-    IterableFilter<String> stackFilter,
+    StackTrace? stack,
+    IterableFilter<String>? stackFilter,
   ) {
     if (stack == null)
       return <DiagnosticsNode>[];
@@ -1178,9 +1176,9 @@ class DiagnosticsStackTrace extends DiagnosticsBlock {
 
 class _FlutterErrorDetailsNode extends DiagnosticableNode<FlutterErrorDetails> {
   _FlutterErrorDetailsNode({
-    String name,
-    @required FlutterErrorDetails value,
-    @required DiagnosticsTreeStyle style,
+    String? name,
+    required FlutterErrorDetails value,
+    required DiagnosticsTreeStyle? style,
   }) : super(
     name: name,
     value: value,
@@ -1188,8 +1186,8 @@ class _FlutterErrorDetailsNode extends DiagnosticableNode<FlutterErrorDetails> {
   );
 
   @override
-  DiagnosticPropertiesBuilder get builder {
-    final DiagnosticPropertiesBuilder builder = super.builder;
+  DiagnosticPropertiesBuilder? get builder {
+    final DiagnosticPropertiesBuilder? builder = super.builder;
     if (builder == null){
       return null;
     }

packages/flutter/lib/src/foundation/basic_types.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:collection';
 
@@ -207,7 +205,7 @@ class _LazyListIterator<E> implements Iterator<E> {
   E get current {
     assert(_index >= 0); // called "current" before "moveNext()"
     if (_index < 0 || _index == _owner._results.length)
-      return null;
+      throw StateError('current can not be call after moveNext has returned false');
     return _owner._results[_index];
   }
 

packages/flutter/lib/src/foundation/binding.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:developer' as developer;
@@ -316,8 +314,8 @@ abstract class BindingBase {
   /// {@macro flutter.foundation.bindingBase.registerServiceExtension}
   @protected
   void registerSignalServiceExtension({
-    @required String name,
-    @required AsyncCallback callback,
+    required String name,
+    required AsyncCallback callback,
   }) {
     assert(name != null);
     assert(callback != null);
@@ -346,9 +344,9 @@ abstract class BindingBase {
   /// {@macro flutter.foundation.bindingBase.registerServiceExtension}
   @protected
   void registerBoolServiceExtension({
-    @required String name,
-    @required AsyncValueGetter<bool> getter,
-    @required AsyncValueSetter<bool> setter,
+    required String name,
+    required AsyncValueGetter<bool> getter,
+    required AsyncValueSetter<bool> setter,
   }) {
     assert(name != null);
     assert(getter != null);
@@ -380,9 +378,9 @@ abstract class BindingBase {
   /// {@macro flutter.foundation.bindingBase.registerServiceExtension}
   @protected
   void registerNumericServiceExtension({
-    @required String name,
-    @required AsyncValueGetter<double> getter,
-    @required AsyncValueSetter<double> setter,
+    required String name,
+    required AsyncValueGetter<double> getter,
+    required AsyncValueSetter<double> setter,
   }) {
     assert(name != null);
     assert(getter != null);
@@ -391,7 +389,7 @@ abstract class BindingBase {
       name: name,
       callback: (Map<String, String> parameters) async {
         if (parameters.containsKey(name)) {
-          await setter(double.parse(parameters[name]));
+          await setter(double.parse(parameters[name]!));
           _postExtensionStateChangedEvent(name, (await getter()).toString());
         }
         return <String, dynamic>{name: (await getter()).toString()};
@@ -442,9 +440,9 @@ abstract class BindingBase {
   /// {@macro flutter.foundation.bindingBase.registerServiceExtension}
   @protected
   void registerStringServiceExtension({
-    @required String name,
-    @required AsyncValueGetter<String> getter,
-    @required AsyncValueSetter<String> setter,
+    required String name,
+    required AsyncValueGetter<String> getter,
+    required AsyncValueSetter<String> setter,
   }) {
     assert(name != null);
     assert(getter != null);
@@ -453,7 +451,7 @@ abstract class BindingBase {
       name: name,
       callback: (Map<String, String> parameters) async {
         if (parameters.containsKey('value')) {
-          await setter(parameters['value']);
+          await setter(parameters['value']!);
           _postExtensionStateChangedEvent(name, await getter());
         }
         return <String, dynamic>{'value': await getter()};
@@ -514,8 +512,8 @@ abstract class BindingBase {
   /// {@endtemplate}
   @protected
   void registerServiceExtension({
-    @required String name,
-    @required ServiceExtensionCallback callback,
+    required String name,
+    required ServiceExtensionCallback callback,
   }) {
     assert(name != null);
     assert(callback != null);
@@ -543,8 +541,8 @@ abstract class BindingBase {
       });
 
       dynamic caughtException;
-      StackTrace caughtStack;
-      Map<String, dynamic> result;
+      StackTrace? caughtStack;
+      late Map<String, dynamic> result;
       try {
         result = await callback(parameters);
       } catch (exception, stack) {

packages/flutter/lib/src/foundation/bitfield.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 '_bitfield_io.dart'
   if (dart.library.html) '_bitfield_web.dart' as _bitfield;
 

packages/flutter/lib/src/foundation/change_notifier.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:meta/meta.dart';
 
 import 'assertions.dart';
@@ -65,7 +63,7 @@ abstract class Listenable {
   /// will lead to memory leaks or exceptions.
   ///
   /// The list may contain nulls; they are ignored.
-  factory Listenable.merge(List<Listenable> listenables) = _MergingListenable;
+  factory Listenable.merge(List<Listenable?> listenables) = _MergingListenable;
 
   /// Register a closure to be called when the object notifies its listeners.
   void addListener(VoidCallback listener);
@@ -100,7 +98,7 @@ abstract class ValueListenable<T> extends Listenable {
 ///
 ///  * [ValueNotifier], which is a [ChangeNotifier] that wraps a single value.
 class ChangeNotifier implements Listenable {
-  ObserverList<VoidCallback> _listeners = ObserverList<VoidCallback>();
+  ObserverList<VoidCallback>? _listeners = ObserverList<VoidCallback>();
 
   bool _debugAssertNotDisposed() {
     assert(() {
@@ -133,7 +131,7 @@ class ChangeNotifier implements Listenable {
   @protected
   bool get hasListeners {
     assert(_debugAssertNotDisposed());
-    return _listeners.isNotEmpty;
+    return _listeners!.isNotEmpty;
   }
 
   /// Register a closure to be called when the object changes.
@@ -142,7 +140,7 @@ class ChangeNotifier implements Listenable {
   @override
   void addListener(VoidCallback listener) {
     assert(_debugAssertNotDisposed());
-    _listeners.add(listener);
+    _listeners!.add(listener);
   }
 
   /// Remove a previously registered closure from the list of closures that are
@@ -167,7 +165,7 @@ class ChangeNotifier implements Listenable {
   @override
   void removeListener(VoidCallback listener) {
     assert(_debugAssertNotDisposed());
-    _listeners.remove(listener);
+    _listeners!.remove(listener);
   }
 
   /// Discards any resources used by the object. After this is called, the
@@ -202,10 +200,10 @@ class ChangeNotifier implements Listenable {
   void notifyListeners() {
     assert(_debugAssertNotDisposed());
     if (_listeners != null) {
-      final List<VoidCallback> localListeners = List<VoidCallback>.from(_listeners);
+      final List<VoidCallback> localListeners = List<VoidCallback>.from(_listeners!);
       for (final VoidCallback listener in localListeners) {
         try {
-          if (_listeners.contains(listener))
+          if (_listeners!.contains(listener))
             listener();
         } catch (exception, stack) {
           FlutterError.reportError(FlutterErrorDetails(
@@ -230,18 +228,18 @@ class ChangeNotifier implements Listenable {
 class _MergingListenable extends Listenable {
   _MergingListenable(this._children);
 
-  final List<Listenable> _children;
+  final List<Listenable?> _children;
 
   @override
   void addListener(VoidCallback listener) {
-    for (final Listenable child in _children) {
+    for (final Listenable? child in _children) {
       child?.addListener(listener);
     }
   }
 
   @override
   void removeListener(VoidCallback listener) {
-    for (final Listenable child in _children) {
+    for (final Listenable? child in _children) {
       child?.removeListener(listener);
     }
   }

packages/flutter/lib/src/foundation/collections.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
-
 // TODO(ianh): These should be on the Set and List classes themselves.
 
 /// Compares two sets for deep equality.
@@ -21,7 +19,7 @@
 ///
 ///  * [listEquals], which does something similar for lists.
 ///  * [mapEquals], which does something similar for maps.
-bool setEquals<T>(Set<T> a, Set<T> b) {
+bool setEquals<T>(Set<T>? a, Set<T>? b) {
   if (a == null)
     return b == null;
   if (b == null || a.length != b.length)
@@ -50,7 +48,7 @@ bool setEquals<T>(Set<T> a, Set<T> b) {
 ///
 ///  * [setEquals], which does something similar for sets.
 ///  * [mapEquals], which does something similar for maps.
-bool listEquals<T>(List<T> a, List<T> b) {
+bool listEquals<T>(List<T>? a, List<T>? b) {
   if (a == null)
     return b == null;
   if (b == null || a.length != b.length)
@@ -79,7 +77,7 @@ bool listEquals<T>(List<T> a, List<T> b) {
 ///
 ///  * [setEquals], which does something similar for sets.
 ///  * [listEquals], which does something similar for lists.
-bool mapEquals<T, U>(Map<T, U> a, Map<T, U> b) {
+bool mapEquals<T, U>(Map<T, U>? a, Map<T, U>? b) {
   if (a == null)
     return b == null;
   if (b == null || a.length != b.length)
@@ -145,8 +143,8 @@ const int _kMergeSortLimit = 32;
 void mergeSort<T>(
   List<T> list, {
   int start = 0,
-  int end,
-  int Function(T, T) compare,
+  int? end,
+  int Function(T, T)? compare,
 }) {
   end ??= list.length;
   compare ??= _defaultCompare<T>();
@@ -156,7 +154,7 @@ void mergeSort<T>(
     return;
   }
   if (length < _kMergeSortLimit) {
-    _insertionSort(list, compare: compare, start: start, end: end);
+    _insertionSort<T>(list, compare: compare, start: start, end: end);
     return;
   }
   // Special case the first split instead of directly calling _mergeSort,
@@ -168,11 +166,11 @@ void mergeSort<T>(
   final int firstLength = middle - start;
   final int secondLength = end - middle;
   // secondLength is always the same as firstLength, or one greater.
-  final List<T> scratchSpace = List<T>(secondLength);
-  _mergeSort(list, compare, middle, end, scratchSpace, 0);
+  final List<T?> scratchSpace = List<T?>.filled(secondLength, null, growable: false);
+  _mergeSort<T>(list, compare, middle, end, scratchSpace, 0);
   final int firstTarget = end - firstLength;
-  _mergeSort(list, compare, start, middle, list, firstTarget);
-  _merge(compare, list, firstTarget, end, scratchSpace, 0, secondLength, list, start);
+  _mergeSort<T>(list, compare, start, middle, list, firstTarget);
+  _merge<T>(compare, list, firstTarget, end, scratchSpace, 0, secondLength, list, start);
 }
 
 /// Returns a [Comparator] that asserts that its first argument is comparable.
@@ -202,9 +200,9 @@ Comparator<T> _defaultCompare<T>() {
 /// they started in.
 void _insertionSort<T>(
   List<T> list, {
-  int Function(T, T) compare,
+  int Function(T, T)? compare,
   int start = 0,
-  int end,
+  int? end,
 }) {
   // If the same method could have both positional and named optional
   // parameters, this should be (list, [start, end], {compare}).
@@ -238,7 +236,7 @@ void _movingInsertionSort<T>(
   int Function(T, T) compare,
   int start,
   int end,
-  List<T> target,
+  List<T?> target,
   int targetOffset,
 ) {
   final int length = end - start;
@@ -252,7 +250,7 @@ void _movingInsertionSort<T>(
     int max = targetOffset + i;
     while (min < max) {
       final int mid = min + ((max - min) >> 1);
-      if (compare(element, target[mid]) < 0) {
+      if (compare(element, target[mid] as T) < 0) {
         max = mid;
       } else {
         min = mid + 1;
@@ -275,12 +273,12 @@ void _mergeSort<T>(
     int Function(T, T) compare,
     int start,
     int end,
-    List<T> target,
+    List<T?> target,
     int targetOffset,
     ) {
   final int length = end - start;
   if (length < _kMergeSortLimit) {
-    _movingInsertionSort(list, compare, start, end, target, targetOffset);
+    _movingInsertionSort<T>(list, compare, start, end, target, targetOffset);
     return;
   }
   final int middle = start + (length >> 1);
@@ -289,11 +287,11 @@ void _mergeSort<T>(
   // Here secondLength >= firstLength (differs by at most one).
   final int targetMiddle = targetOffset + firstLength;
   // Sort the second half into the end of the target area.
-  _mergeSort(list, compare, middle, end, target, targetMiddle);
+  _mergeSort<T>(list, compare, middle, end, target, targetMiddle);
   // Sort the first half into the end of the source area.
-  _mergeSort(list, compare, start, middle, list, middle);
+  _mergeSort<T>(list, compare, start, middle, list, middle);
   // Merge the two parts into the target area.
-  _merge(
+  _merge<T>(
     compare,
     list,
     middle,
@@ -318,10 +316,10 @@ void _merge<T>(
   List<T> firstList,
   int firstStart,
   int firstEnd,
-  List<T> secondList,
+  List<T?> secondList,
   int secondStart,
   int secondEnd,
-  List<T> target,
+  List<T?> target,
   int targetOffset,
 ) {
   // No empty lists reaches here.
@@ -330,7 +328,7 @@ void _merge<T>(
   int cursor1 = firstStart;
   int cursor2 = secondStart;
   T firstElement = firstList[cursor1++];
-  T secondElement = secondList[cursor2++];
+  T secondElement = secondList[cursor2++] as T;
   while (true) {
     if (compare(firstElement, secondElement) <= 0) {
       target[targetOffset++] = firstElement;
@@ -342,7 +340,7 @@ void _merge<T>(
     } else {
       target[targetOffset++] = secondElement;
       if (cursor2 != secondEnd) {
-        secondElement = secondList[cursor2++];
+        secondElement = secondList[cursor2++] as T;
         continue;
       }
       // Second list empties first. Flushing first list here.

packages/flutter/lib/src/foundation/consolidate_response.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';
@@ -25,7 +23,7 @@ import 'dart:typed_data';
 /// until the request has been fully processed).
 ///
 /// This is used in [consolidateHttpClientResponseBytes].
-typedef BytesReceivedCallback = void Function(int cumulative, int total);
+typedef BytesReceivedCallback = void Function(int cumulative, int? total);
 
 /// Efficiently converts the response body of an [HttpClientResponse] into a
 /// [Uint8List].
@@ -48,14 +46,14 @@ typedef BytesReceivedCallback = void Function(int cumulative, int total);
 Future<Uint8List> consolidateHttpClientResponseBytes(
   HttpClientResponse response, {
   bool autoUncompress = true,
-  BytesReceivedCallback onBytesReceived,
+  BytesReceivedCallback? onBytesReceived,
 }) {
   assert(autoUncompress != null);
   final Completer<Uint8List> completer = Completer<Uint8List>.sync();
 
   final _OutputBuffer output = _OutputBuffer();
   ByteConversionSink sink = output;
-  int expectedContentLength = response.contentLength;
+  int? expectedContentLength = response.contentLength;
   if (expectedContentLength == -1)
     expectedContentLength = null;
   switch (response.compressionState) {
@@ -76,7 +74,7 @@ Future<Uint8List> consolidateHttpClientResponseBytes(
   }
 
   int bytesReceived = 0;
-  StreamSubscription<List<int>> subscription;
+  late final StreamSubscription<List<int>> subscription;
   subscription = response.listen((List<int> chunk) {
     sink.add(chunk);
     if (onBytesReceived != null) {
@@ -98,14 +96,14 @@ Future<Uint8List> consolidateHttpClientResponseBytes(
 }
 
 class _OutputBuffer extends ByteConversionSinkBase {
-  List<List<int>> _chunks = <List<int>>[];
+  List<List<int>>? _chunks = <List<int>>[];
   int _contentLength = 0;
-  Uint8List _bytes;
+  Uint8List? _bytes;
 
   @override
   void add(List<int> chunk) {
     assert(_bytes == null);
-    _chunks.add(chunk);
+    _chunks!.add(chunk);
     _contentLength += chunk.length;
   }
 
@@ -117,8 +115,8 @@ class _OutputBuffer extends ByteConversionSinkBase {
     }
     _bytes = Uint8List(_contentLength);
     int offset = 0;
-    for (final List<int> chunk in _chunks) {
-      _bytes.setRange(offset, offset + chunk.length, chunk);
+    for (final List<int> chunk in _chunks!) {
+      _bytes!.setRange(offset, offset + chunk.length, chunk);
       offset += chunk.length;
     }
     _chunks = null;
@@ -126,6 +124,6 @@ class _OutputBuffer extends ByteConversionSinkBase {
 
   Uint8List get bytes {
     assert(_bytes != null);
-    return _bytes;
+    return _bytes!;
   }
 }

packages/flutter/lib/src/foundation/constants.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
-
 /// A constant that is true if the application was compiled in release mode.
 ///
 /// More specifically, this is a constant that is true if the application was

packages/flutter/lib/src/foundation/debug.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:ui' as ui show Brightness;
 
@@ -54,7 +52,7 @@ bool debugInstrumentationEnabled = false;
 ///  * [Timeline], which is used to record synchronous tracing events for
 ///    visualization in Chrome's tracing format. This method does not
 ///    implicitly add any timeline events.
-Future<T> debugInstrumentAction<T>(String description, Future<T> action()) {
+Future<T> debugInstrumentAction<T>(String description, Future<T> action()) async {
   bool instrument = false;
   assert(() {
     instrument = debugInstrumentationEnabled;
@@ -62,10 +60,12 @@ Future<T> debugInstrumentAction<T>(String description, Future<T> action()) {
   }());
   if (instrument) {
     final Stopwatch stopwatch = Stopwatch()..start();
-    return action().whenComplete(() {
+    try {
+      return await action();
+    } finally {
       stopwatch.stop();
       debugPrint('Action "$description" took ${stopwatch.elapsed}');
-    });
+    }
   } else {
     return action();
   }
@@ -87,17 +87,17 @@ const Map<String, String> timelineArgumentsIndicatingLandmarkEvent = <String, St
 /// Configure [debugFormatDouble] using [num.toStringAsPrecision].
 ///
 /// Defaults to null, which uses the default logic of [debugFormatDouble].
-int debugDoublePrecision;
+int? debugDoublePrecision;
 
 /// Formats a double to have standard formatting.
 ///
 /// This behavior can be overridden by [debugDoublePrecision].
-String debugFormatDouble(double value) {
+String debugFormatDouble(double? value) {
   if (value == null) {
     return 'null';
   }
   if (debugDoublePrecision != null) {
-    return value.toStringAsPrecision(debugDoublePrecision);
+    return value.toStringAsPrecision(debugDoublePrecision!);
   }
   return value.toStringAsFixed(1);
 }
@@ -109,4 +109,4 @@ String debugFormatDouble(double value) {
 ///
 ///  * [WidgetsApp], which uses the [debugBrightnessOverride] setting in debug mode
 ///    to construct a [MediaQueryData].
-ui.Brightness debugBrightnessOverride;
+ui.Brightness? debugBrightnessOverride;

packages/flutter/lib/src/foundation/diagnostics.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:math' as math;
 
 import 'package:meta/meta.dart';
@@ -203,13 +201,13 @@ class TextTreeConfiguration {
   ///
   /// All of the arguments must not be null.
   TextTreeConfiguration({
-    @required this.prefixLineOne,
-    @required this.prefixOtherLines,
-    @required this.prefixLastChildLineOne,
-    @required this.prefixOtherLinesRootNode,
-    @required this.linkCharacter,
-    @required this.propertyPrefixIfChildren,
-    @required this.propertyPrefixNoChildren,
+    required this.prefixLineOne,
+    required this.prefixOtherLines,
+    required this.prefixLastChildLineOne,
+    required this.prefixOtherLinesRootNode,
+    required this.linkCharacter,
+    required this.propertyPrefixIfChildren,
+    required this.propertyPrefixNoChildren,
     this.lineBreak = '\n',
     this.lineBreakProperties = true,
     this.afterName = ':',
@@ -786,8 +784,8 @@ enum _WordWrapParseMode { inSpace, inWord, atBreak }
 /// [prefixOtherLines].
 class _PrefixedStringBuilder {
   _PrefixedStringBuilder({
-    @required this.prefixLineOne,
-    @required String prefixOtherLines,
+    required this.prefixLineOne,
+    required String? prefixOtherLines,
     this.wrapWidth}) :
       _prefixOtherLines = prefixOtherLines;
 
@@ -798,24 +796,24 @@ class _PrefixedStringBuilder {
   ///
   /// The prefix can be modified while the string is being built in which case
   /// subsequent lines will be added with the modified prefix.
-  String get prefixOtherLines => _nextPrefixOtherLines ?? _prefixOtherLines;
-  String _prefixOtherLines;
-  set prefixOtherLines(String prefix) {
+  String? get prefixOtherLines => _nextPrefixOtherLines ?? _prefixOtherLines;
+  String? _prefixOtherLines;
+  set prefixOtherLines(String? prefix) {
     _prefixOtherLines = prefix;
     _nextPrefixOtherLines = null;
   }
 
-  String _nextPrefixOtherLines;
-  void incrementPrefixOtherLines(String suffix, {@required bool updateCurrentLine}) {
+  String? _nextPrefixOtherLines;
+  void incrementPrefixOtherLines(String suffix, {required bool updateCurrentLine}) {
     if (_currentLine.isEmpty || updateCurrentLine) {
-      _prefixOtherLines = prefixOtherLines + suffix;
+      _prefixOtherLines = prefixOtherLines! + suffix;
       _nextPrefixOtherLines = null;
     } else {
-      _nextPrefixOtherLines = prefixOtherLines + suffix;
+      _nextPrefixOtherLines = prefixOtherLines! + suffix;
     }
   }
 
-  final int wrapWidth;
+  final int? wrapWidth;
 
   /// Buffer containing lines that have already been completely laid out.
   final StringBuffer _buffer = StringBuffer();
@@ -827,7 +825,7 @@ class _PrefixedStringBuilder {
 
   /// Whether the string being built already has more than 1 line.
   bool get requiresMultipleLines => _numLines > 1 || (_numLines == 1 && _currentLine.isNotEmpty) ||
-      (_currentLine.length + _getCurrentPrefix(true).length > wrapWidth);
+      (_currentLine.length + _getCurrentPrefix(true)!.length > wrapWidth!);
 
   bool get isCurrentLineEmpty => _currentLine.isEmpty;
 
@@ -850,9 +848,9 @@ class _PrefixedStringBuilder {
     final Iterable<String> lines = _wordWrapLine(
       text,
       _wrappableRanges,
-      wrapWidth,
-      startOffset: firstLine ? prefixLineOne.length : _prefixOtherLines.length,
-      otherLineOffset: firstLine ? _prefixOtherLines.length : _prefixOtherLines.length,
+      wrapWidth!,
+      startOffset: firstLine ? prefixLineOne.length : _prefixOtherLines!.length,
+      otherLineOffset: firstLine ? _prefixOtherLines!.length : _prefixOtherLines!.length,
     );
     int i = 0;
     final int length = lines.length;
@@ -887,8 +885,8 @@ class _PrefixedStringBuilder {
     bool addPrefix = false;
     int index = 0;
     _WordWrapParseMode mode = _WordWrapParseMode.inSpace;
-    int lastWordStart;
-    int lastWordEnd;
+    late int lastWordStart;
+    int? lastWordEnd;
     int start = 0;
 
     int currentChunk = 0;
@@ -1000,8 +998,8 @@ class _PrefixedStringBuilder {
 
   void _writeLine(
     String line, {
-    @required bool includeLineBreak,
-    @required bool firstLine,
+    required bool includeLineBreak,
+    required bool firstLine,
   }) {
     line = '${_getCurrentPrefix(firstLine)}$line';
     _buffer.write(line.trimRight());
@@ -1010,7 +1008,7 @@ class _PrefixedStringBuilder {
     _numLines++;
   }
 
-  String _getCurrentPrefix(bool firstLine) {
+  String? _getCurrentPrefix(bool firstLine) {
     return _buffer.isEmpty ? prefixLineOne : (firstLine ? _prefixOtherLines : _prefixOtherLines);
   }
 
@@ -1035,7 +1033,7 @@ class _PrefixedStringBuilder {
   /// Finishes the current line with a stretched version of text.
   void writeStretched(String text, int targetLineLength) {
     write(text);
-    final int currentLineLength = _currentLine.length + _getCurrentPrefix(_buffer.isEmpty).length;
+    final int currentLineLength = _currentLine.length + _getCurrentPrefix(_buffer.isEmpty)!.length;
     assert (_currentLine.length > 0);
     final int targetLength = targetLineLength - currentLineLength;
     if (targetLength > 0) {
@@ -1063,7 +1061,7 @@ class _NoDefaultValue {
 /// Marker object indicating that a [DiagnosticsNode] has no default value.
 const _NoDefaultValue kNoDefaultValue = _NoDefaultValue();
 
-bool _isSingleLine(DiagnosticsTreeStyle style) {
+bool _isSingleLine(DiagnosticsTreeStyle? style) {
   return style == DiagnosticsTreeStyle.singleLine;
 }
 
@@ -1107,11 +1105,11 @@ class TextTreeRenderer {
   /// parent to the child should be consistent with the parent's style as the
   /// single line style does not provide any meaningful style for how children
   /// should be connected to their parents.
-  TextTreeConfiguration _childTextConfiguration(
+  TextTreeConfiguration? _childTextConfiguration(
     DiagnosticsNode child,
     TextTreeConfiguration textStyle,
   ) {
-    final DiagnosticsTreeStyle childStyle = child?.style;
+    final DiagnosticsTreeStyle? childStyle = child.style;
     return (_isSingleLine(childStyle) || childStyle == DiagnosticsTreeStyle.errorProperty) ? textStyle : child.textTreeConfiguration;
   }
 
@@ -1119,8 +1117,8 @@ class TextTreeRenderer {
   String render(
     DiagnosticsNode node, {
     String prefixLineOne = '',
-    String prefixOtherLines,
-    TextTreeConfiguration parentConfiguration,
+    String? prefixOtherLines,
+    TextTreeConfiguration? parentConfiguration,
   }) {
     if (kReleaseMode) {
       return '';
@@ -1136,17 +1134,17 @@ class TextTreeRenderer {
   String _debugRender(
     DiagnosticsNode node, {
     String prefixLineOne = '',
-    String prefixOtherLines,
-    TextTreeConfiguration parentConfiguration,
+    String? prefixOtherLines,
+    TextTreeConfiguration? parentConfiguration,
   }) {
     final bool isSingleLine = _isSingleLine(node.style) && parentConfiguration?.lineBreakProperties != true;
     prefixOtherLines ??= prefixLineOne;
     if (node.linePrefix != null) {
-      prefixLineOne += node.linePrefix;
-      prefixOtherLines += node.linePrefix;
+      prefixLineOne += node.linePrefix!;
+      prefixOtherLines += node.linePrefix!;
     }
 
-    final TextTreeConfiguration config = node.textTreeConfiguration;
+    final TextTreeConfiguration config = node.textTreeConfiguration!;
     if (prefixOtherLines.isEmpty)
       prefixOtherLines += config.prefixOtherLinesRootNode;
 
@@ -1192,14 +1190,14 @@ class TextTreeRenderer {
 
     List<DiagnosticsNode> children = node.getChildren();
 
-    String description = node.toDescription(parentConfiguration: parentConfiguration);
+    String? description = node.toDescription(parentConfiguration: parentConfiguration);
     if (config.beforeName.isNotEmpty) {
       builder.write(config.beforeName);
     }
     final bool wrapName = !isSingleLine && node.allowNameWrap;
     final bool wrapDescription = !isSingleLine && node.allowWrap;
     final bool uppercaseTitle = node.style == DiagnosticsTreeStyle.error;
-    String name = node.name;
+    String? name = node.name;
     if (uppercaseTitle) {
       name = name?.toUpperCase();
     }
@@ -1244,7 +1242,7 @@ class TextTreeRenderer {
       }
     }
     if (config.suffixLineOne.isNotEmpty) {
-      builder.writeStretched(config.suffixLineOne, builder.wrapWidth);
+      builder.writeStretched(config.suffixLineOne, builder.wrapWidth!);
     }
 
     final Iterable<DiagnosticsNode> propertiesIterable = node.getProperties().where(
@@ -1287,7 +1285,7 @@ class TextTreeRenderer {
         properties.isEmpty &&
         children.isEmpty &&
         prefixLineOne.isNotEmpty) {
-      builder.write(node.emptyBodyDescription);
+      builder.write(node.emptyBodyDescription!);
       if (config.lineBreakProperties)
         builder.write(config.lineBreak);
     }
@@ -1297,7 +1295,7 @@ class TextTreeRenderer {
       if (i > 0)
         builder.write(config.propertySeparator);
 
-      final TextTreeConfiguration propertyStyle = property.textTreeConfiguration;
+      final TextTreeConfiguration propertyStyle = property.textTreeConfiguration!;
       if (_isSingleLine(property.style)) {
         // We have to treat single line properties slightly differently to deal
         // with cases where a single line properties output may not have single
@@ -1337,7 +1335,7 @@ class TextTreeRenderer {
     if (children.isEmpty &&
         config.addBlankLineIfNoChildren &&
         builder.requiresMultipleLines &&
-        builder.prefixOtherLines.trimRight().isNotEmpty
+        builder.prefixOtherLines!.trimRight().isNotEmpty
     ) {
       builder.write(config.lineBreak);
     }
@@ -1345,7 +1343,7 @@ class TextTreeRenderer {
     if (children.isNotEmpty && config.showChildren) {
       if (config.isBlankLineBetweenPropertiesAndChildren &&
           properties.isNotEmpty &&
-          children.first.textTreeConfiguration.isBlankLineBetweenPropertiesAndChildren) {
+          children.first.textTreeConfiguration!.isBlankLineBetweenPropertiesAndChildren) {
         builder.write(config.lineBreak);
       }
 
@@ -1354,7 +1352,7 @@ class TextTreeRenderer {
       for (int i = 0; i < children.length; i++) {
         final DiagnosticsNode child = children[i];
         assert(child != null);
-        final TextTreeConfiguration childConfig = _childTextConfiguration(child, config);
+        final TextTreeConfiguration childConfig = _childTextConfiguration(child, config)!;
         if (i == children.length - 1) {
           final String lastChildPrefixLineOne = '$prefixChildrenRaw${childConfig.prefixLastChildLineOne}';
           final String childPrefixOtherLines = '$prefixChildrenRaw${childConfig.childLinkSpace}${childConfig.prefixOtherLines}';
@@ -1370,13 +1368,13 @@ class TextTreeRenderer {
             if (childConfig.mandatoryFooter.isNotEmpty) {
               builder.writeStretched(
                 childConfig.mandatoryFooter,
-                math.max(builder.wrapWidth, _wrapWidthProperties + childPrefixOtherLines.length),
+                math.max(builder.wrapWidth!, _wrapWidthProperties + childPrefixOtherLines.length),
               );
             }
             builder.write(config.lineBreak);
           }
         } else {
-          final TextTreeConfiguration nextChildStyle = _childTextConfiguration(children[i + 1], config);
+          final TextTreeConfiguration nextChildStyle = _childTextConfiguration(children[i + 1], config)!;
           final String childPrefixLineOne = '$prefixChildrenRaw${childConfig.prefixLineOne}';
           final String childPrefixOtherLines ='$prefixChildrenRaw${nextChildStyle.linkCharacter}${childConfig.prefixOtherLines}';
           builder.writeRawLines(render(
@@ -1391,7 +1389,7 @@ class TextTreeRenderer {
             if (childConfig.mandatoryFooter.isNotEmpty) {
               builder.writeStretched(
                 childConfig.mandatoryFooter,
-                math.max(builder.wrapWidth, _wrapWidthProperties + childPrefixOtherLines.length),
+                math.max(builder.wrapWidth!, _wrapWidthProperties + childPrefixOtherLines.length),
               );
             }
             builder.write(config.lineBreak);
@@ -1400,7 +1398,7 @@ class TextTreeRenderer {
       }
     }
     if (parentConfiguration == null && config.mandatoryFooter.isNotEmpty) {
-      builder.writeStretched(config.mandatoryFooter, builder.wrapWidth);
+      builder.writeStretched(config.mandatoryFooter, builder.wrapWidth!);
       builder.write(config.lineBreak);
     }
     return builder.build();
@@ -1423,7 +1421,7 @@ abstract class DiagnosticsNode {
   /// The [style], [showName], and [showSeparator] arguments must not
   /// be null.
   DiagnosticsNode({
-    @required this.name,
+    required this.name,
     this.style,
     this.showName = true,
     this.showSeparator = true,
@@ -1472,7 +1470,7 @@ abstract class DiagnosticsNode {
   /// (see [showSeparator]).
   ///
   /// The name will be omitted if the [showName] property is false.
-  final String name;
+  final String? name;
 
   /// Returns a description with a short summary of the node itself not
   /// including children or properties.
@@ -1480,7 +1478,7 @@ abstract class DiagnosticsNode {
   /// `parentConfiguration` specifies how the parent is rendered as text art.
   /// For example, if the parent does not line break between properties, the
   /// description of a property should also be a single line if possible.
-  String toDescription({ TextTreeConfiguration parentConfiguration });
+  String? toDescription({ TextTreeConfiguration? parentConfiguration });
 
   /// Whether to show a separator between [name] and description.
   ///
@@ -1514,16 +1512,16 @@ abstract class DiagnosticsNode {
   final bool showName;
 
   /// Prefix to include at the start of each line
-  final String linePrefix;
+  final String? linePrefix;
 
   /// Description to show if the node has no displayed properties or children.
-  String get emptyBodyDescription => null;
+  String? get emptyBodyDescription => null;
 
   /// The actual object this is diagnostics data for.
-  Object get value;
+  Object? get value;
 
   /// Hint for how the node should be displayed.
-  final DiagnosticsTreeStyle style;
+  final DiagnosticsTreeStyle? style;
 
   /// Whether to wrap text on onto multiple lines or not.
   bool get allowWrap => false;
@@ -1561,11 +1559,11 @@ abstract class DiagnosticsNode {
   ///    by this method and interactive tree views in the Flutter IntelliJ
   ///    plugin.
   @mustCallSuper
-  Map<String, Object> toJsonMap(DiagnosticsSerializationDelegate delegate) {
-    Map<String, Object> result = <String, Object>{};
+  Map<String, Object?> toJsonMap(DiagnosticsSerializationDelegate delegate) {
+    Map<String, Object?> result = <String, Object?>{};
     assert(() {
       final bool hasChildren = getChildren().isNotEmpty;
-      result = <String, Object>{
+      result = <String, Object?>{
         'description': toDescription(),
         'type': runtimeType.toString(),
         if (name != null)
@@ -1579,7 +1577,7 @@ abstract class DiagnosticsNode {
         if (emptyBodyDescription != null)
           'emptyBodyDescription': emptyBodyDescription,
         if (style != DiagnosticsTreeStyle.sparse)
-          'style': describeEnum(style),
+          'style': describeEnum(style!),
         if (allowTruncate)
           'allowTruncate': allowTruncate,
         if (hasChildren)
@@ -1614,21 +1612,21 @@ abstract class DiagnosticsNode {
   ///
   /// The provided `nodes` may be properties or children of the `parent`
   /// [DiagnosticsNode].
-  static List<Map<String, Object>> toJsonList(
-    List<DiagnosticsNode> nodes,
-    DiagnosticsNode parent,
+  static List<Map<String, Object?>> toJsonList(
+    List<DiagnosticsNode>? nodes,
+    DiagnosticsNode? parent,
     DiagnosticsSerializationDelegate delegate,
   ) {
     bool truncated = false;
     if (nodes == null)
-      return const <Map<String, Object>>[];
+      return const <Map<String, Object?>>[];
     final int originalNodeCount = nodes.length;
     nodes = delegate.truncateNodesList(nodes, parent);
     if (nodes.length != originalNodeCount) {
       nodes.add(DiagnosticsNode.message('...'));
       truncated = true;
     }
-    final List<Map<String, Object>> json = nodes.map<Map<String, Object>>((DiagnosticsNode node) {
+    final List<Map<String, Object?>> json = nodes.map<Map<String, Object?>>((DiagnosticsNode node) {
       return node.toJsonMap(delegate.delegateForNode(node));
     }).toList();
     if (truncated)
@@ -1650,7 +1648,7 @@ abstract class DiagnosticsNode {
   /// not print at all.
   @override
   String toString({
-    TextTreeConfiguration parentConfiguration,
+    TextTreeConfiguration? parentConfiguration,
     DiagnosticLevel minLevel = DiagnosticLevel.info,
   }) {
     String result = super.toString();
@@ -1661,11 +1659,11 @@ abstract class DiagnosticsNode {
         result = toStringDeep(
             parentConfiguration: parentConfiguration, minLevel: minLevel);
       } else {
-        final String description = toDescription(
-            parentConfiguration: parentConfiguration);
+        String? description = toDescription(parentConfiguration: parentConfiguration);
         assert(description != null);
+        description = description!;
 
-        if (name == null || name.isEmpty || !showName) {
+        if (name == null || name!.isEmpty || !showName) {
           result = description;
         } else {
           result = description.contains('\n') ? '$name$_separator\n$description'
@@ -1680,9 +1678,9 @@ abstract class DiagnosticsNode {
   /// Returns a configuration specifying how this object should be rendered
   /// as text art.
   @protected
-  TextTreeConfiguration get textTreeConfiguration {
+  TextTreeConfiguration? get textTreeConfiguration {
     assert(style != null);
-    switch (style) {
+    switch (style!) {
       case DiagnosticsTreeStyle.none:
         return null;
       case DiagnosticsTreeStyle.dense:
@@ -1710,7 +1708,6 @@ abstract class DiagnosticsNode {
       case DiagnosticsTreeStyle.flat:
         return flatTextConfiguration;
     }
-    return null;
   }
 
   /// Returns a string representation of this node and its descendants.
@@ -1735,8 +1732,8 @@ abstract class DiagnosticsNode {
   ///    children.
   String toStringDeep({
     String prefixLineOne = '',
-    String prefixOtherLines,
-    TextTreeConfiguration parentConfiguration,
+    String? prefixOtherLines,
+    TextTreeConfiguration? parentConfiguration,
     DiagnosticLevel minLevel = DiagnosticLevel.debug,
   }) {
     String result = '';
@@ -1817,13 +1814,13 @@ class StringProperty extends DiagnosticsProperty<String> {
   /// The [showName], [quoted], [style], and [level] arguments must not be null.
   StringProperty(
     String name,
-    String value, {
-    String description,
-    String tooltip,
+    String? value, {
+    String? description,
+    String? tooltip,
     bool showName = true,
-    Object defaultValue = kNoDefaultValue,
+    Object? defaultValue = kNoDefaultValue,
     this.quoted = true,
-    String ifEmpty,
+    String? ifEmpty,
     DiagnosticsTreeStyle style = DiagnosticsTreeStyle.singleLine,
     DiagnosticLevel level = DiagnosticLevel.info,
   }) : assert(showName != null),
@@ -1846,15 +1843,15 @@ class StringProperty extends DiagnosticsProperty<String> {
   final bool quoted;
 
   @override
-  Map<String, Object> toJsonMap(DiagnosticsSerializationDelegate delegate) {
-    final Map<String, Object> json = super.toJsonMap(delegate);
+  Map<String, Object?> toJsonMap(DiagnosticsSerializationDelegate delegate) {
+    final Map<String, Object?> json = super.toJsonMap(delegate);
     json['quoted'] = quoted;
     return json;
   }
 
   @override
-  String valueToString({ TextTreeConfiguration parentConfiguration }) {
-    String text = _description ?? value;
+  String valueToString({ TextTreeConfiguration? parentConfiguration }) {
+    String? text = _description ?? value;
     if (parentConfiguration != null &&
         !parentConfiguration.lineBreakProperties &&
         text != null) {
@@ -1868,7 +1865,7 @@ class StringProperty extends DiagnosticsProperty<String> {
       // An empty value would not appear empty after being surrounded with
       // quotes so we have to handle this case separately.
       if (ifEmpty != null && text.isEmpty)
-        return ifEmpty;
+        return ifEmpty!;
       return '"$text"';
     }
     return text.toString();
@@ -1878,12 +1875,12 @@ class StringProperty extends DiagnosticsProperty<String> {
 abstract class _NumProperty<T extends num> extends DiagnosticsProperty<T> {
   _NumProperty(
     String name,
-    T value, {
-    String ifNull,
+    T? value, {
+    String? ifNull,
     this.unit,
     bool showName = true,
-    Object defaultValue = kNoDefaultValue,
-    String tooltip,
+    Object? defaultValue = kNoDefaultValue,
+    String? tooltip,
     DiagnosticsTreeStyle style = DiagnosticsTreeStyle.singleLine,
     DiagnosticLevel level = DiagnosticLevel.info,
   }) : super(
@@ -1900,11 +1897,11 @@ abstract class _NumProperty<T extends num> extends DiagnosticsProperty<T> {
   _NumProperty.lazy(
     String name,
     ComputePropertyValueCallback<T> computeValue, {
-    String ifNull,
+    String? ifNull,
     this.unit,
     bool showName = true,
-    Object defaultValue = kNoDefaultValue,
-    String tooltip,
+    Object? defaultValue = kNoDefaultValue,
+    String? tooltip,
     DiagnosticsTreeStyle style = DiagnosticsTreeStyle.singleLine,
     DiagnosticLevel level = DiagnosticLevel.info,
   }) : super.lazy(
@@ -1919,8 +1916,8 @@ abstract class _NumProperty<T extends num> extends DiagnosticsProperty<T> {
   );
 
   @override
-  Map<String, Object> toJsonMap(DiagnosticsSerializationDelegate delegate) {
-    final Map<String, Object> json = super.toJsonMap(delegate);
+  Map<String, Object?> toJsonMap(DiagnosticsSerializationDelegate delegate) {
+    final Map<String, Object?> json = super.toJsonMap(delegate);
     if (unit != null)
       json['unit'] = unit;
 
@@ -1933,13 +1930,13 @@ abstract class _NumProperty<T extends num> extends DiagnosticsProperty<T> {
   /// Unit must be acceptable to display immediately after a number with no
   /// spaces. For example: 'physical pixels per logical pixel' should be a
   /// [tooltip] not a [unit].
-  final String unit;
+  final String? unit;
 
   /// String describing just the numeric [value] without a unit suffix.
   String numberToString();
 
   @override
-  String valueToString({ TextTreeConfiguration parentConfiguration }) {
+  String valueToString({ TextTreeConfiguration? parentConfiguration }) {
     if (value == null)
       return value.toString();
 
@@ -1955,11 +1952,11 @@ class DoubleProperty extends _NumProperty<double> {
   /// The [showName], [style], and [level] arguments must not be null.
   DoubleProperty(
     String name,
-    double value, {
-    String ifNull,
-    String unit,
-    String tooltip,
-    Object defaultValue = kNoDefaultValue,
+    double? value, {
+    String? ifNull,
+    String? unit,
+    String? tooltip,
+    Object? defaultValue = kNoDefaultValue,
     bool showName = true,
     DiagnosticsTreeStyle style = DiagnosticsTreeStyle.singleLine,
     DiagnosticLevel level = DiagnosticLevel.info,
@@ -1987,11 +1984,11 @@ class DoubleProperty extends _NumProperty<double> {
   DoubleProperty.lazy(
     String name,
     ComputePropertyValueCallback<double> computeValue, {
-    String ifNull,
+    String? ifNull,
     bool showName = true,
-    String unit,
-    String tooltip,
-    Object defaultValue = kNoDefaultValue,
+    String? unit,
+    String? tooltip,
+    Object? defaultValue = kNoDefaultValue,
     DiagnosticLevel level = DiagnosticLevel.info,
   }) : assert(showName != null),
        assert(level != null),
@@ -2019,11 +2016,11 @@ class IntProperty extends _NumProperty<int> {
   /// The [showName], [style], and [level] arguments must not be null.
   IntProperty(
     String name,
-    int value, {
-    String ifNull,
+    int? value, {
+    String? ifNull,
     bool showName = true,
-    String unit,
-    Object defaultValue = kNoDefaultValue,
+    String? unit,
+    Object? defaultValue = kNoDefaultValue,
     DiagnosticsTreeStyle style = DiagnosticsTreeStyle.singleLine,
     DiagnosticLevel level = DiagnosticLevel.info,
   }) : assert(showName != null),
@@ -2057,10 +2054,10 @@ class PercentProperty extends DoubleProperty {
   PercentProperty(
     String name,
     double fraction, {
-    String ifNull,
+    String? ifNull,
     bool showName = true,
-    String tooltip,
-    String unit,
+    String? tooltip,
+    String? unit,
     DiagnosticLevel level  = DiagnosticLevel.info,
   }) : assert(showName != null),
        assert(level != null),
@@ -2075,7 +2072,7 @@ class PercentProperty extends DoubleProperty {
   );
 
   @override
-  String valueToString({ TextTreeConfiguration parentConfiguration }) {
+  String valueToString({ TextTreeConfiguration? parentConfiguration }) {
     if (value == null)
       return value.toString();
     return unit != null ? '${numberToString()} $unit' : numberToString();
@@ -2083,9 +2080,10 @@ class PercentProperty extends DoubleProperty {
 
   @override
   String numberToString() {
-    if (value == null)
+    final double? v = value;
+    if (v == null)
       return value.toString();
-    return '${(value.clamp(0.0, 1.0) * 100.0).toStringAsFixed(1)}%';
+    return '${(v.clamp(0.0, 1.0) * 100.0).toStringAsFixed(1)}%';
   }
 }
 
@@ -2137,11 +2135,11 @@ class FlagProperty extends DiagnosticsProperty<bool> {
   /// The [showName] and [level] arguments must not be null.
   FlagProperty(
     String name, {
-    @required bool value,
+    required bool? value,
     this.ifTrue,
     this.ifFalse,
     bool showName = false,
-    Object defaultValue,
+    Object? defaultValue,
     DiagnosticLevel level = DiagnosticLevel.info,
   }) : assert(showName != null),
        assert(level != null),
@@ -2155,8 +2153,8 @@ class FlagProperty extends DiagnosticsProperty<bool> {
        );
 
   @override
-  Map<String, Object> toJsonMap(DiagnosticsSerializationDelegate delegate) {
-    final Map<String, Object> json = super.toJsonMap(delegate);
+  Map<String, Object?> toJsonMap(DiagnosticsSerializationDelegate delegate) {
+    final Map<String, Object?> json = super.toJsonMap(delegate);
     if (ifTrue != null)
       json['ifTrue'] = ifTrue;
     if (ifFalse != null)
@@ -2169,22 +2167,22 @@ class FlagProperty extends DiagnosticsProperty<bool> {
   ///
   /// If not specified and [value] equals true the property's priority [level]
   /// will be [DiagnosticLevel.hidden].
-  final String ifTrue;
+  final String? ifTrue;
 
   /// Description to use if the property value is false.
   ///
   /// If not specified and [value] equals false, the property's priority [level]
   /// will be [DiagnosticLevel.hidden].
-  final String ifFalse;
+  final String? ifFalse;
 
   @override
-  String valueToString({ TextTreeConfiguration parentConfiguration }) {
+  String valueToString({ TextTreeConfiguration? parentConfiguration }) {
     if (value == true) {
       if (ifTrue != null)
-        return ifTrue;
+        return ifTrue!;
     } else if (value == false) {
       if (ifFalse != null)
-        return ifFalse;
+        return ifFalse!;
     }
     return super.valueToString(parentConfiguration: parentConfiguration);
   }
@@ -2233,10 +2231,10 @@ class IterableProperty<T> extends DiagnosticsProperty<Iterable<T>> {
   /// The [style], [showName], [showSeparator], and [level] arguments must not be null.
   IterableProperty(
     String name,
-    Iterable<T> value, {
-    Object defaultValue = kNoDefaultValue,
-    String ifNull,
-    String ifEmpty = '[]',
+    Iterable<T>? value, {
+    Object? defaultValue = kNoDefaultValue,
+    String? ifNull,
+    String? ifEmpty = '[]',
     DiagnosticsTreeStyle style = DiagnosticsTreeStyle.singleLine,
     bool showName = true,
     bool showSeparator = true,
@@ -2258,14 +2256,14 @@ class IterableProperty<T> extends DiagnosticsProperty<Iterable<T>> {
   );
 
   @override
-  String valueToString({TextTreeConfiguration parentConfiguration}) {
+  String valueToString({TextTreeConfiguration? parentConfiguration}) {
     if (value == null)
       return value.toString();
 
-    if (value.isEmpty)
+    if (value!.isEmpty)
       return ifEmpty ?? '[]';
 
-    final Iterable<String> formattedValues = value.map((T v) {
+    final Iterable<String> formattedValues = value!.map((T v) {
       if (T == double && v is double) {
         return debugFormatDouble(v);
       } else {
@@ -2291,16 +2289,16 @@ class IterableProperty<T> extends DiagnosticsProperty<Iterable<T>> {
   /// null.
   @override
   DiagnosticLevel get level {
-    if (ifEmpty == null && value != null && value.isEmpty && super.level != DiagnosticLevel.hidden)
+    if (ifEmpty == null && value != null && value!.isEmpty && super.level != DiagnosticLevel.hidden)
       return DiagnosticLevel.fine;
     return super.level;
   }
 
   @override
-  Map<String, Object> toJsonMap(DiagnosticsSerializationDelegate delegate) {
-    final Map<String, Object> json = super.toJsonMap(delegate);
+  Map<String, Object?> toJsonMap(DiagnosticsSerializationDelegate delegate) {
+    final Map<String, Object?> json = super.toJsonMap(delegate);
     if (value != null) {
-      json['values'] = value.map<String>((T value) => value.toString()).toList();
+      json['values'] = value!.map<String>((T value) => value.toString()).toList();
     }
     return json;
   }
@@ -2321,8 +2319,8 @@ class EnumProperty<T> extends DiagnosticsProperty<T> {
   /// The [level] argument must also not be null.
   EnumProperty(
     String name,
-    T value, {
-    Object defaultValue = kNoDefaultValue,
+    T? value, {
+    Object? defaultValue = kNoDefaultValue,
     DiagnosticLevel level  = DiagnosticLevel.info,
   }) : assert(level != null),
        super (
@@ -2333,10 +2331,10 @@ class EnumProperty<T> extends DiagnosticsProperty<T> {
   );
 
   @override
-  String valueToString({ TextTreeConfiguration parentConfiguration }) {
+  String valueToString({ TextTreeConfiguration? parentConfiguration }) {
     if (value == null)
       return value.toString();
-    return describeEnum(value);
+    return describeEnum(value!);
   }
 }
 
@@ -2371,9 +2369,9 @@ class ObjectFlagProperty<T> extends DiagnosticsProperty<T> {
   /// least one of [ifPresent] and [ifNull] must not be null.
   ObjectFlagProperty(
     String name,
-    T value, {
+    T? value, {
     this.ifPresent,
-    String ifNull,
+    String? ifNull,
     bool showName = false,
     DiagnosticLevel level  = DiagnosticLevel.info,
   }) : assert(ifPresent != null || ifNull != null),
@@ -2395,7 +2393,7 @@ class ObjectFlagProperty<T> extends DiagnosticsProperty<T> {
   /// The [name] and [level] arguments must not be null.
   ObjectFlagProperty.has(
     String name,
-    T value, {
+    T? value, {
     DiagnosticLevel level = DiagnosticLevel.info,
   }) : assert(name != null),
        assert(level != null),
@@ -2412,16 +2410,16 @@ class ObjectFlagProperty<T> extends DiagnosticsProperty<T> {
   /// If the property [value] is not null and [ifPresent] is null, the
   /// [level] for the property is [DiagnosticLevel.hidden] and the description
   /// from superclass is used.
-  final String ifPresent;
+  final String? ifPresent;
 
   @override
-  String valueToString({ TextTreeConfiguration parentConfiguration }) {
+  String valueToString({ TextTreeConfiguration? parentConfiguration }) {
     if (value != null) {
       if (ifPresent != null)
-        return ifPresent;
+        return ifPresent!;
     } else {
       if (ifNull != null)
-        return ifNull;
+        return ifNull!;
     }
     return super.valueToString(parentConfiguration: parentConfiguration);
   }
@@ -2452,8 +2450,8 @@ class ObjectFlagProperty<T> extends DiagnosticsProperty<T> {
   }
 
   @override
-  Map<String, Object> toJsonMap(DiagnosticsSerializationDelegate delegate) {
-    final Map<String, Object> json = super.toJsonMap(delegate);
+  Map<String, Object?> toJsonMap(DiagnosticsSerializationDelegate delegate) {
+    final Map<String, Object?> json = super.toJsonMap(delegate);
     if (ifPresent != null)
       json['ifPresent'] = ifPresent;
     return json;
@@ -2490,7 +2488,7 @@ class FlagsSummary<T> extends DiagnosticsProperty<Map<String, T>> {
   FlagsSummary(
     String name,
     Map<String, T> value, {
-    String ifEmpty,
+    String? ifEmpty,
     bool showName = true,
     bool showSeparator = true,
     DiagnosticLevel level  = DiagnosticLevel.info,
@@ -2508,10 +2506,13 @@ class FlagsSummary<T> extends DiagnosticsProperty<Map<String, T>> {
        );
 
   @override
-  String valueToString({TextTreeConfiguration parentConfiguration}) {
+  Map<String, T> get value => super.value as Map<String, T>;
+
+  @override
+  String valueToString({TextTreeConfiguration? parentConfiguration}) {
     assert(value != null);
     if (!_hasNonNullEntry() && ifEmpty != null)
-      return ifEmpty;
+      return ifEmpty!;
 
     final Iterable<String> formattedValues = _formattedValues();
     if (parentConfiguration != null && !parentConfiguration.lineBreakProperties) {
@@ -2536,14 +2537,14 @@ class FlagsSummary<T> extends DiagnosticsProperty<Map<String, T>> {
   }
 
   @override
-  Map<String, Object> toJsonMap(DiagnosticsSerializationDelegate delegate) {
-    final Map<String, Object> json = super.toJsonMap(delegate);
+  Map<String, Object?> toJsonMap(DiagnosticsSerializationDelegate delegate) {
+    final Map<String, Object?> json = super.toJsonMap(delegate);
     if (value.isNotEmpty)
       json['values'] = _formattedValues().toList();
     return json;
   }
 
-  bool _hasNonNullEntry() => value.values.any((Object o) => o != null);
+  bool _hasNonNullEntry() => value.values.any((T o) => o != null);
 
   // An iterable of each entry's description in [value].
   //
@@ -2585,17 +2586,17 @@ class DiagnosticsProperty<T> extends DiagnosticsNode {
   /// level. For example, if the property value is null and [missingIfNull] is
   /// true, [level] is raised to [DiagnosticLevel.warning].
   DiagnosticsProperty(
-    String name,
-    T value, {
-    String description,
-    String ifNull,
+    String? name,
+    T? value, {
+    String? description,
+    String? ifNull,
     this.ifEmpty,
     bool showName = true,
     bool showSeparator = true,
     this.defaultValue = kNoDefaultValue,
     this.tooltip,
     this.missingIfNull = false,
-    String linePrefix,
+    String? linePrefix,
     this.expandableValue = false,
     this.allowWrap = true,
     this.allowNameWrap = true,
@@ -2632,10 +2633,10 @@ class DiagnosticsProperty<T> extends DiagnosticsNode {
   /// level. For example, if calling `computeValue` throws an exception, [level]
   /// will always return [DiagnosticLevel.error].
   DiagnosticsProperty.lazy(
-    String name,
+    String? name,
     ComputePropertyValueCallback<T> computeValue, {
-    String description,
-    String ifNull,
+    String? description,
+    String? ifNull,
     this.ifEmpty,
     bool showName = true,
     bool showSeparator = true,
@@ -2666,7 +2667,7 @@ class DiagnosticsProperty<T> extends DiagnosticsNode {
          style: style,
        );
 
-  final String _description;
+  final String? _description;
 
   /// Whether to expose properties and children of the value as properties and
   /// children.
@@ -2679,9 +2680,9 @@ class DiagnosticsProperty<T> extends DiagnosticsNode {
   final bool allowNameWrap;
 
   @override
-  Map<String, Object> toJsonMap(DiagnosticsSerializationDelegate delegate) {
-    final T v = value;
-    List<Map<String, Object>> properties;
+  Map<String, Object?> toJsonMap(DiagnosticsSerializationDelegate delegate) {
+    final T? v = value;
+    List<Map<String, Object?>>? properties;
     if (delegate.expandPropertyValues && delegate.includeProperties && v is Diagnosticable && getProperties().isEmpty) {
       // Exclude children for expanded nodes to avoid cycles.
       delegate = delegate.copyWith(subtreeDepth: 0, includeProperties: false);
@@ -2691,7 +2692,7 @@ class DiagnosticsProperty<T> extends DiagnosticsNode {
         delegate,
       );
     }
-    final Map<String, Object> json = super.toJsonMap(delegate);
+    final Map<String, Object?> json = super.toJsonMap(delegate);
     if (properties != null) {
       json['properties'] = properties;
     }
@@ -2732,28 +2733,28 @@ class DiagnosticsProperty<T> extends DiagnosticsNode {
   /// `parentConfiguration` specifies how the parent is rendered as text art.
   /// For example, if the parent places all properties on one line, the value
   /// of the property should be displayed without line breaks if possible.
-  String valueToString({ TextTreeConfiguration parentConfiguration }) {
-    final T v = value;
+  String valueToString({ TextTreeConfiguration? parentConfiguration }) {
+    final T? v = value;
     // DiagnosticableTree values are shown using the shorter toStringShort()
     // instead of the longer toString() because the toString() for a
     // DiagnosticableTree value is likely too large to be useful.
-    return (v is DiagnosticableTree ? v.toStringShort() : v.toString()) ?? '';
+    return v is DiagnosticableTree ? v.toStringShort() : v.toString();
   }
 
   @override
-  String toDescription({ TextTreeConfiguration parentConfiguration }) {
+  String toDescription({ TextTreeConfiguration? parentConfiguration }) {
     if (_description != null)
-      return _addTooltip(_description);
+      return _addTooltip(_description!);
 
     if (exception != null)
       return 'EXCEPTION (${exception.runtimeType})';
 
     if (ifNull != null && value == null)
-      return _addTooltip(ifNull);
+      return _addTooltip(ifNull!);
 
     String result = valueToString(parentConfiguration: parentConfiguration);
     if (result.isEmpty && ifEmpty != null)
-      result = ifEmpty;
+      result = ifEmpty!;
     return _addTooltip(result);
   }
 
@@ -2768,10 +2769,10 @@ class DiagnosticsProperty<T> extends DiagnosticsNode {
   }
 
   /// Description if the property [value] is null.
-  final String ifNull;
+  final String? ifNull;
 
   /// Description if the property description would otherwise be empty.
-  final String ifEmpty;
+  final String? ifEmpty;
 
   /// Optional tooltip typically describing the property.
   ///
@@ -2779,7 +2780,7 @@ class DiagnosticsProperty<T> extends DiagnosticsNode {
   ///
   /// If present, the tooltip is added in parenthesis after the raw value when
   /// generating the string description.
-  final String tooltip;
+  final String? tooltip;
 
   /// Whether a [value] of null causes the property to have [level]
   /// [DiagnosticLevel.warning] warning that the property is missing a [value].
@@ -2809,21 +2810,21 @@ class DiagnosticsProperty<T> extends DiagnosticsNode {
   ///
   ///  * [valueToString], which converts the property value to a string.
   @override
-  T get value {
+  T? get value {
     _maybeCacheValue();
     return _value;
   }
 
-  T _value;
+  T? _value;
 
   bool _valueComputed;
 
-  Object _exception;
+  Object? _exception;
 
   /// Exception thrown if accessing the property [value] threw an exception.
   ///
   /// Returns null if computing the property value did not throw an exception.
-  Object get exception {
+  Object? get exception {
     _maybeCacheValue();
     return _exception;
   }
@@ -2835,7 +2836,7 @@ class DiagnosticsProperty<T> extends DiagnosticsNode {
     _valueComputed = true;
     assert(_computeValue != null);
     try {
-      _value = _computeValue();
+      _value = _computeValue!();
     } catch (exception) {
       _exception = exception;
       _value = null;
@@ -2847,7 +2848,7 @@ class DiagnosticsProperty<T> extends DiagnosticsNode {
   /// value is uninteresting.
   ///
   /// [defaultValue] has type [T] or is [kNoDefaultValue].
-  final Object defaultValue;
+  final Object? defaultValue;
 
   final DiagnosticLevel _defaultLevel;
 
@@ -2879,12 +2880,12 @@ class DiagnosticsProperty<T> extends DiagnosticsNode {
     return _defaultLevel;
   }
 
-  final ComputePropertyValueCallback<T> _computeValue;
+  final ComputePropertyValueCallback<T>? _computeValue;
 
   @override
   List<DiagnosticsNode> getProperties() {
     if (expandableValue) {
-      final T object = value;
+      final T? object = value;
       if (object is DiagnosticsNode) {
         return object.getProperties();
       }
@@ -2898,7 +2899,7 @@ class DiagnosticsProperty<T> extends DiagnosticsNode {
   @override
   List<DiagnosticsNode> getChildren() {
     if (expandableValue) {
-      final T object = value;
+      final T? object = value;
       if (object is DiagnosticsNode) {
         return object.getChildren();
       }
@@ -2917,9 +2918,9 @@ class DiagnosticableNode<T extends Diagnosticable> extends DiagnosticsNode {
   ///
   /// The [value] argument must not be null.
   DiagnosticableNode({
-    String name,
-    @required this.value,
-    @required DiagnosticsTreeStyle style,
+    String? name,
+    required this.value,
+    required DiagnosticsTreeStyle? style,
   }) : assert(value != null),
        super(
          name: name,
@@ -2929,19 +2930,19 @@ class DiagnosticableNode<T extends Diagnosticable> extends DiagnosticsNode {
   @override
   final T value;
 
-  DiagnosticPropertiesBuilder _cachedBuilder;
+  DiagnosticPropertiesBuilder? _cachedBuilder;
 
   /// Retrieve the [DiagnosticPropertiesBuilder] of current node.
   ///
   /// It will cache the result to prevent duplicate operation.
-  DiagnosticPropertiesBuilder get builder {
+  DiagnosticPropertiesBuilder? get builder {
     if (kReleaseMode) {
       return null;
     } else {
       assert(() {
         if (_cachedBuilder == null) {
           _cachedBuilder = DiagnosticPropertiesBuilder();
-          value?.debugFillProperties(_cachedBuilder);
+          value.debugFillProperties(_cachedBuilder!);
         }
         return true;
       }());
@@ -2951,14 +2952,14 @@ class DiagnosticableNode<T extends Diagnosticable> extends DiagnosticsNode {
 
   @override
   DiagnosticsTreeStyle get style {
-    return kReleaseMode ? DiagnosticsTreeStyle.none : super.style ?? builder.defaultDiagnosticsTreeStyle;
+    return kReleaseMode ? DiagnosticsTreeStyle.none : super.style ?? builder!.defaultDiagnosticsTreeStyle;
   }
 
   @override
-  String get emptyBodyDescription => (kReleaseMode || kProfileMode) ? '' : builder.emptyBodyDescription;
+  String? get emptyBodyDescription => (kReleaseMode || kProfileMode) ? '' : builder!.emptyBodyDescription;
 
   @override
-  List<DiagnosticsNode> getProperties() => (kReleaseMode || kProfileMode) ? const <DiagnosticsNode>[] : builder.properties;
+  List<DiagnosticsNode> getProperties() => (kReleaseMode || kProfileMode) ? const <DiagnosticsNode>[] : builder!.properties;
 
   @override
   List<DiagnosticsNode> getChildren() {
@@ -2966,7 +2967,7 @@ class DiagnosticableNode<T extends Diagnosticable> extends DiagnosticsNode {
   }
 
   @override
-  String toDescription({ TextTreeConfiguration parentConfiguration }) {
+  String toDescription({ TextTreeConfiguration? parentConfiguration }) {
     String result = '';
     assert(() {
       result = value.toStringShort();
@@ -2980,9 +2981,9 @@ class DiagnosticableNode<T extends Diagnosticable> extends DiagnosticsNode {
 class DiagnosticableTreeNode extends DiagnosticableNode<DiagnosticableTree> {
   /// Creates a [DiagnosticableTreeNode].
   DiagnosticableTreeNode({
-    String name,
-    @required DiagnosticableTree value,
-    @required DiagnosticsTreeStyle style,
+    String? name,
+    required DiagnosticableTree value,
+    required DiagnosticsTreeStyle? style,
   }) : super(
          name: name,
          value: value,
@@ -2990,16 +2991,12 @@ class DiagnosticableTreeNode extends DiagnosticableNode<DiagnosticableTree> {
        );
 
   @override
-  List<DiagnosticsNode> getChildren() {
-    if (value != null)
-      return value.debugDescribeChildren();
-    return const <DiagnosticsNode>[];
-  }
+  List<DiagnosticsNode> getChildren() => value.debugDescribeChildren();
 }
 
 /// Returns a 5 character long hexadecimal string generated from
 /// [Object.hashCode]'s 20 least-significant bits.
-String shortHash(Object object) {
+String shortHash(Object? object) {
   return object.hashCode.toUnsigned(20).toRadixString(16).padLeft(5, '0');
 }
 
@@ -3012,7 +3009,7 @@ String shortHash(Object object) {
 ///    distinguish instances of the same class (hash collisions are
 ///    possible, but rare enough that its use in debug output is useful).
 ///  * [Object.runtimeType], the [Type] of an object.
-String describeIdentity(Object object) => '${objectRuntimeType(object, '<optimized out>')}#${shortHash(object)}';
+String describeIdentity(Object? object) => '${objectRuntimeType(object, '<optimized out>')}#${shortHash(object)}';
 
 // This method exists as a workaround for https://github.com/dart-lang/sdk/issues/30021
 /// Returns a short description of an enum value.
@@ -3064,7 +3061,7 @@ class DiagnosticPropertiesBuilder {
   DiagnosticsTreeStyle defaultDiagnosticsTreeStyle = DiagnosticsTreeStyle.sparse;
 
   /// Description to show if the node has no displayed properties or children.
-  String emptyBodyDescription;
+  String? emptyBodyDescription;
 }
 
 // Examples can assume:
@@ -3105,7 +3102,7 @@ mixin Diagnosticable {
 
   @override
   String toString({ DiagnosticLevel minLevel = DiagnosticLevel.info }) {
-    String fullString;
+    String? fullString;
     assert(() {
       fullString = toDiagnosticsNode(style: DiagnosticsTreeStyle.singleLine).toString(minLevel: minLevel);
       return true;
@@ -3122,7 +3119,7 @@ mixin Diagnosticable {
   /// Typically the [style] argument is only specified to indicate an atypical
   /// relationship between the parent and the node. For example, pass
   /// [DiagnosticsTreeStyle.offstage] to indicate that a node is offstage.
-  DiagnosticsNode toDiagnosticsNode({ String name, DiagnosticsTreeStyle style }) {
+  DiagnosticsNode toDiagnosticsNode({ String? name, DiagnosticsTreeStyle? style }) {
     return DiagnosticableNode<Diagnosticable>(
       name: name,
       value: this,
@@ -3377,14 +3374,14 @@ abstract class DiagnosticableTree with Diagnosticable {
   ///
   ///  * [toString], for a brief description of the object.
   ///  * [toStringDeep], for a description of the subtree rooted at this object.
-  String toStringShallow({
+  String? toStringShallow({
     String joiner = ', ',
     DiagnosticLevel minLevel = DiagnosticLevel.debug,
   }) {
     if (kReleaseMode) {
       return toString();
     }
-    String shallowString;
+    String? shallowString;
     assert(() {
       final StringBuffer result = StringBuffer();
       result.write(toString());
@@ -3421,7 +3418,7 @@ abstract class DiagnosticableTree with Diagnosticable {
   ///    children.
   String toStringDeep({
     String prefixLineOne = '',
-    String prefixOtherLines,
+    String? prefixOtherLines,
     DiagnosticLevel minLevel = DiagnosticLevel.debug,
   }) {
     return toDiagnosticsNode().toStringDeep(prefixLineOne: prefixLineOne, prefixOtherLines: prefixOtherLines, minLevel: minLevel);
@@ -3431,7 +3428,7 @@ abstract class DiagnosticableTree with Diagnosticable {
   String toStringShort() => describeIdentity(this);
 
   @override
-  DiagnosticsNode toDiagnosticsNode({ String name, DiagnosticsTreeStyle style }) {
+  DiagnosticsNode toDiagnosticsNode({ String? name, DiagnosticsTreeStyle? style }) {
     return DiagnosticableTreeNode(
       name: name,
       value: this,
@@ -3470,14 +3467,14 @@ mixin DiagnosticableTreeMixin implements DiagnosticableTree {
   }
 
   @override
-  String toStringShallow({
+  String? toStringShallow({
     String joiner = ', ',
     DiagnosticLevel minLevel = DiagnosticLevel.debug,
   }) {
     if (kReleaseMode) {
       return toString();
     }
-    String shallowString;
+    String? shallowString;
     assert(() {
       final StringBuffer result = StringBuffer();
       result.write(toStringShort());
@@ -3497,7 +3494,7 @@ mixin DiagnosticableTreeMixin implements DiagnosticableTree {
   @override
   String toStringDeep({
     String prefixLineOne = '',
-    String prefixOtherLines,
+    String? prefixOtherLines,
     DiagnosticLevel minLevel = DiagnosticLevel.debug,
   }) {
     return toDiagnosticsNode().toStringDeep(prefixLineOne: prefixLineOne, prefixOtherLines: prefixOtherLines, minLevel: minLevel);
@@ -3507,7 +3504,7 @@ mixin DiagnosticableTreeMixin implements DiagnosticableTree {
   String toStringShort() => describeIdentity(this);
 
   @override
-  DiagnosticsNode toDiagnosticsNode({ String name, DiagnosticsTreeStyle style }) {
+  DiagnosticsNode toDiagnosticsNode({ String? name, DiagnosticsTreeStyle? style }) {
     return DiagnosticableTreeNode(
       name: name,
       value: this,
@@ -3531,13 +3528,13 @@ class DiagnosticsBlock extends DiagnosticsNode {
   /// Creates a diagnostic with properties specified by [properties] and
   /// children specified by [children].
   DiagnosticsBlock({
-    String name,
+    String? name,
     DiagnosticsTreeStyle style = DiagnosticsTreeStyle.whitespace,
     bool showName = true,
     bool showSeparator = true,
-    String linePrefix,
+    String? linePrefix,
     this.value,
-    String description,
+    String? description,
     this.level = DiagnosticLevel.info,
     this.allowTruncate = false,
     List<DiagnosticsNode> children = const<DiagnosticsNode>[],
@@ -3558,9 +3555,9 @@ class DiagnosticsBlock extends DiagnosticsNode {
 
   @override
   final DiagnosticLevel level;
-  final String _description;
+  final String? _description;
   @override
-  final Object value;
+  final Object? value;
 
   @override
   final bool allowTruncate;
@@ -3572,7 +3569,7 @@ class DiagnosticsBlock extends DiagnosticsNode {
   List<DiagnosticsNode> getProperties() => _properties;
 
   @override
-  String toDescription({TextTreeConfiguration parentConfiguration}) => _description;
+  String? toDescription({TextTreeConfiguration? parentConfiguration}) => _description;
 }
 
 /// A delegate that configures how a hierarchy of [DiagnosticsNode]s should be
@@ -3597,7 +3594,7 @@ abstract class DiagnosticsSerializationDelegate {
   ///
   /// This method is called for every [DiagnosticsNode] that's included in
   /// the serialization.
-  Map<String, Object> additionalNodeProperties(DiagnosticsNode node);
+  Map<String, Object?> additionalNodeProperties(DiagnosticsNode node);
 
   /// Filters the list of [DiagnosticsNode]s that will be included as children
   /// for the given `owner` node.
@@ -3635,7 +3632,7 @@ abstract class DiagnosticsSerializationDelegate {
   /// result that are not included in the current serialization.
   ///
   /// By default, `nodes` is returned as-is.
-  List<DiagnosticsNode> truncateNodesList(List<DiagnosticsNode> nodes, DiagnosticsNode owner);
+  List<DiagnosticsNode> truncateNodesList(List<DiagnosticsNode> nodes, DiagnosticsNode? owner);
 
   /// Returns the [DiagnosticsSerializationDelegate] to be used
   /// for adding the provided [DiagnosticsNode] to the serialization.
@@ -3689,8 +3686,8 @@ class _DefaultDiagnosticsSerializationDelegate implements DiagnosticsSerializati
   });
 
   @override
-  Map<String, Object> additionalNodeProperties(DiagnosticsNode node) {
-    return const <String, Object>{};
+  Map<String, Object?> additionalNodeProperties(DiagnosticsNode node) {
+    return const <String, Object?>{};
   }
 
   @override
@@ -3718,12 +3715,12 @@ class _DefaultDiagnosticsSerializationDelegate implements DiagnosticsSerializati
   final int subtreeDepth;
 
   @override
-  List<DiagnosticsNode> truncateNodesList(List<DiagnosticsNode> nodes, DiagnosticsNode owner) {
+  List<DiagnosticsNode> truncateNodesList(List<DiagnosticsNode> nodes, DiagnosticsNode? owner) {
     return nodes;
   }
 
   @override
-  DiagnosticsSerializationDelegate copyWith({int subtreeDepth, bool includeProperties}) {
+  DiagnosticsSerializationDelegate copyWith({int? subtreeDepth, bool? includeProperties}) {
     return _DefaultDiagnosticsSerializationDelegate(
       subtreeDepth: subtreeDepth ?? this.subtreeDepth,
       includeProperties: includeProperties ?? this.includeProperties,

packages/flutter/lib/src/foundation/isolates.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 '_isolates_io.dart'
@@ -20,7 +18,7 @@ import '_isolates_io.dart'
 typedef ComputeCallback<Q, R> = FutureOr<R> Function(Q message);
 
 // The signature of [compute].
-typedef _ComputeImpl = Future<R> Function<Q, R>(ComputeCallback<Q, R> callback, Q message, { String debugLabel });
+typedef _ComputeImpl = Future<R> Function<Q, R>(ComputeCallback<Q, R> callback, Q message, { String? debugLabel });
 
 /// Spawn an isolate, run `callback` on that isolate, passing it `message`, and
 /// (eventually) return the value returned by `callback`.

packages/flutter/lib/src/foundation/key.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:ui' show hashValues;
 
 import 'package:meta/meta.dart';

packages/flutter/lib/src/foundation/licenses.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 'package:meta/meta.dart' show visibleForTesting;
@@ -49,7 +47,7 @@ abstract class LicenseEntry {
   const LicenseEntry();
 
   /// The names of the packages that this license entry applies to.
-  Iterable<String> get packages;
+  Iterable<String>? get packages;
 
   /// The paragraphs of the license, each as a [LicenseParagraph] consisting of
   /// a string and some formatting information. Paragraphs can include newline
@@ -125,7 +123,7 @@ class LicenseEntryWithLineBreaks extends LicenseEntry {
   const LicenseEntryWithLineBreaks(this.packages, this.text);
 
   @override
-  final List<String> packages;
+  final List<String>? packages;
 
   /// The text of the license.
   ///
@@ -148,7 +146,7 @@ class LicenseEntryWithLineBreaks extends LicenseEntry {
     int currentPosition = 0;
     int lastLineIndent = 0;
     int currentLineIndent = 0;
-    int currentParagraphIndentation;
+    int? currentParagraphIndentation;
     _LicenseEntryWithLineBreaksParserState state = _LicenseEntryWithLineBreaksParserState.beforeParagraph;
     final List<String> lines = <String>[];
 
@@ -160,7 +158,7 @@ class LicenseEntryWithLineBreaks extends LicenseEntry {
     LicenseParagraph getParagraph() {
       assert(lines.isNotEmpty);
       assert(currentParagraphIndentation != null);
-      final LicenseParagraph result = LicenseParagraph(lines.join(' '), currentParagraphIndentation);
+      final LicenseParagraph result = LicenseParagraph(lines.join(' '), currentParagraphIndentation!);
       assert(result.text.trimLeft() == result.text);
       assert(result.text.isNotEmpty);
       lines.clear();
@@ -295,7 +293,7 @@ class LicenseRegistry {
   // ignore: unused_element
   LicenseRegistry._();
 
-  static List<LicenseEntryCollector> _collectors;
+  static List<LicenseEntryCollector>? _collectors;
 
   /// Adds licenses to the registry.
   ///
@@ -306,7 +304,7 @@ class LicenseRegistry {
   /// licenses, the closure will not be called.
   static void addLicense(LicenseEntryCollector collector) {
     _collectors ??= <LicenseEntryCollector>[];
-    _collectors.add(collector);
+    _collectors!.add(collector);
   }
 
   /// Returns the licenses that have been registered.
@@ -315,7 +313,7 @@ class LicenseRegistry {
   static Stream<LicenseEntry> get licenses async* {
     if (_collectors == null)
       return;
-    for (final LicenseEntryCollector collector in _collectors)
+    for (final LicenseEntryCollector collector in _collectors!)
       yield* collector();
   }
 

packages/flutter/lib/src/foundation/node.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:meta/meta.dart';
 
 // This file gets mutated by //dev/devicelab/bin/tasks/flutter_test_performance.dart
@@ -71,8 +69,8 @@ class AbstractNode {
   /// The owner for this node (null if unattached).
   ///
   /// The entire subtree that this node belongs to will have the same owner.
-  Object get owner => _owner;
-  Object _owner;
+  Object? get owner => _owner;
+  Object? _owner;
 
   /// Whether this node is in a tree whose root is attached to something.
   ///
@@ -107,12 +105,12 @@ class AbstractNode {
   void detach() {
     assert(_owner != null);
     _owner = null;
-    assert(parent == null || attached == parent.attached);
+    assert(parent == null || attached == parent!.attached);
   }
 
   /// The parent of this node in the tree.
-  AbstractNode get parent => _parent;
-  AbstractNode _parent;
+  AbstractNode? get parent => _parent;
+  AbstractNode? _parent;
 
   /// Mark the given node as being a child of this node.
   ///
@@ -125,13 +123,13 @@ class AbstractNode {
     assert(() {
       AbstractNode node = this;
       while (node.parent != null)
-        node = node.parent;
+        node = node.parent!;
       assert(node != child); // indicates we are about to create a cycle
       return true;
     }());
     child._parent = this;
     if (attached)
-      child.attach(_owner);
+      child.attach(_owner!);
     redepthChild(child);
   }
 

packages/flutter/lib/src/foundation/object.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
-
 /// Framework code should use this method in favor of calling `toString` on
 /// [Object.runtimeType].
 ///
@@ -11,7 +9,7 @@
 /// negatively impact performance. If asserts are enabled, this method will
 /// return `object.runtimeType.toString()`; otherwise, it will return the
 /// [optimizedValue], which must be a simple constant string.
-String objectRuntimeType(Object object, String optimizedValue) {
+String objectRuntimeType(Object? object, String optimizedValue) {
   assert(() {
     optimizedValue = object.runtimeType.toString();
     return true;

packages/flutter/lib/src/foundation/observer_list.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:collection';
 
 /// A list optimized for the observer pattern when there are small numbers of
@@ -29,7 +27,7 @@ import 'dart:collection';
 class ObserverList<T> extends Iterable<T> {
   final List<T> _list = <T>[];
   bool _isDirty = false;
-  HashSet<T> _set;
+  late final HashSet<T> _set = HashSet<T>();
 
   /// Adds an item to the end of this list.
   ///
@@ -46,21 +44,17 @@ class ObserverList<T> extends Iterable<T> {
   /// Returns whether the item was present in the list.
   bool remove(T item) {
     _isDirty = true;
-    _set?.clear(); // Clear the set so that we don't leak items.
+    _set.clear(); // Clear the set so that we don't leak items.
     return _list.remove(item);
   }
 
   @override
-  bool contains(Object element) {
+  bool contains(Object? element) {
     if (_list.length < 3)
       return _list.contains(element);
 
     if (_isDirty) {
-      if (_set == null) {
-        _set = HashSet<T>.from(_list);
-      } else {
       _set.addAll(_list);
-      }
       _isDirty = false;
     }
 
@@ -107,7 +101,7 @@ class HashedObserverList<T> extends Iterable<T> {
   ///
   /// Returns whether the item was present in the list.
   bool remove(T item) {
-    final int value = _map[item];
+    final int? value = _map[item];
     if (value == null) {
       return false;
     }
@@ -120,7 +114,7 @@ class HashedObserverList<T> extends Iterable<T> {
   }
 
   @override
-  bool contains(Object element) => _map.containsKey(element);
+  bool contains(Object? element) => _map.containsKey(element);
 
   @override
   Iterator<T> get iterator => _map.keys.iterator;

packages/flutter/lib/src/foundation/platform.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 '_platform_io.dart'
   if (dart.library.html) '_platform_web.dart' as _platform;
 
@@ -79,4 +77,4 @@ enum TargetPlatform {
 /// button, which will make those widgets unusable since iOS has no such button.
 ///
 /// In general, therefore, this property should not be used in release builds.
-TargetPlatform debugDefaultTargetPlatformOverride;
+TargetPlatform? debugDefaultTargetPlatformOverride;

packages/flutter/lib/src/foundation/print.dart

@@ -2,13 +2,11 @@
 // 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:collection';
 
 /// Signature for [debugPrint] implementations.
-typedef DebugPrintCallback = void Function(String message, { int wrapWidth });
+typedef DebugPrintCallback = void Function(String? message, { int? wrapWidth });
 
 /// Prints a message to the console, which you can access using the "flutter"
 /// tool's "logs" command ("flutter logs").
@@ -32,8 +30,8 @@ DebugPrintCallback debugPrint = debugPrintThrottled;
 
 /// Alternative implementation of [debugPrint] that does not throttle.
 /// Used by tests.
-void debugPrintSynchronously(String message, { int wrapWidth }) {
-  if (wrapWidth != null) {
+void debugPrintSynchronously(String? message, { int? wrapWidth }) {
+  if (message != null && wrapWidth != null) {
     print(message.split('\n').expand<String>((String line) => debugWordWrap(line, wrapWidth)).join('\n'));
   } else {
     print(message);
@@ -42,7 +40,7 @@ void debugPrintSynchronously(String message, { int wrapWidth }) {
 
 /// Implementation of [debugPrint] that throttles messages. This avoids dropping
 /// messages on platforms that rate-limit their logging (for example, Android).
-void debugPrintThrottled(String message, { int wrapWidth }) {
+void debugPrintThrottled(String? message, { int? wrapWidth }) {
   final List<String> messageLines = message?.split('\n') ?? <String>['null'];
   if (wrapWidth != null) {
     _debugPrintBuffer.addAll(messageLines.expand<String>((String line) => debugWordWrap(line, wrapWidth)));
@@ -57,7 +55,7 @@ const int _kDebugPrintCapacity = 12 * 1024;
 const Duration _kDebugPrintPauseTime = Duration(seconds: 1);
 final Queue<String> _debugPrintBuffer = Queue<String>();
 final Stopwatch _debugPrintStopwatch = Stopwatch();
-Completer<void> _debugPrintCompleter;
+Completer<void>? _debugPrintCompleter;
 bool _debugPrintScheduled = false;
 void _debugPrintTask() {
   _debugPrintScheduled = false;
@@ -112,15 +110,15 @@ Iterable<String> debugWordWrap(String message, int width, { String wrapIndent =
     yield message;
     return;
   }
-  final Match prefixMatch = _indentPattern.matchAsPrefix(message);
-  final String prefix = wrapIndent + ' ' * prefixMatch.group(0).length;
+  final Match prefixMatch = _indentPattern.matchAsPrefix(message)!;
+  final String prefix = wrapIndent + ' ' * prefixMatch.group(0)!.length;
   int start = 0;
   int startForLengthCalculations = 0;
   bool addPrefix = false;
   int index = prefix.length;
   _WordWrapParseMode mode = _WordWrapParseMode.inSpace;
-  int lastWordStart;
-  int lastWordEnd;
+  late int lastWordStart;
+  int? lastWordEnd;
   while (true) {
     switch (mode) {
       case _WordWrapParseMode.inSpace: // at start of break point (or start of line); can't break until next break

packages/flutter/lib/src/foundation/serialization.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:typed_data';
 
 import 'package:typed_data/typed_buffers.dart' show Uint8Buffer;
@@ -16,86 +14,86 @@ import 'package:typed_data/typed_buffers.dart' show Uint8Buffer;
 /// The byte order used is [Endian.host] throughout.
 class WriteBuffer {
   /// Creates an interface for incrementally building a [ByteData] instance.
-  WriteBuffer() {
-    _buffer = Uint8Buffer();
-    _eightBytes = ByteData(8);
+  WriteBuffer()
+    : _buffer = Uint8Buffer(),
+      _eightBytes = ByteData(8) {
     _eightBytesAsList = _eightBytes.buffer.asUint8List();
   }
 
-  Uint8Buffer _buffer;
-  ByteData _eightBytes;
-  Uint8List _eightBytesAsList;
+  Uint8Buffer? _buffer;
+  final ByteData _eightBytes;
+  late Uint8List _eightBytesAsList;
 
   /// Write a Uint8 into the buffer.
   void putUint8(int byte) {
-    _buffer.add(byte);
+    _buffer!.add(byte);
   }
 
   /// Write a Uint16 into the buffer.
-  void putUint16(int value, {Endian endian}) {
+  void putUint16(int value, {Endian? endian}) {
     _eightBytes.setUint16(0, value, endian ?? Endian.host);
-    _buffer.addAll(_eightBytesAsList, 0, 2);
+    _buffer!.addAll(_eightBytesAsList, 0, 2);
   }
 
   /// Write a Uint32 into the buffer.
-  void putUint32(int value, {Endian endian}) {
+  void putUint32(int value, {Endian? endian}) {
     _eightBytes.setUint32(0, value, endian ?? Endian.host);
-    _buffer.addAll(_eightBytesAsList, 0, 4);
+    _buffer!.addAll(_eightBytesAsList, 0, 4);
   }
 
   /// Write an Int32 into the buffer.
-  void putInt32(int value, {Endian endian}) {
+  void putInt32(int value, {Endian? endian}) {
     _eightBytes.setInt32(0, value, endian ?? Endian.host);
-    _buffer.addAll(_eightBytesAsList, 0, 4);
+    _buffer!.addAll(_eightBytesAsList, 0, 4);
   }
 
   /// Write an Int64 into the buffer.
-  void putInt64(int value, {Endian endian}) {
+  void putInt64(int value, {Endian? endian}) {
     _eightBytes.setInt64(0, value, endian ?? Endian.host);
-    _buffer.addAll(_eightBytesAsList, 0, 8);
+    _buffer!.addAll(_eightBytesAsList, 0, 8);
   }
 
   /// Write an Float64 into the buffer.
-  void putFloat64(double value, {Endian endian}) {
+  void putFloat64(double value, {Endian? endian}) {
     _alignTo(8);
     _eightBytes.setFloat64(0, value, endian ?? Endian.host);
-    _buffer.addAll(_eightBytesAsList);
+    _buffer!.addAll(_eightBytesAsList);
   }
 
   /// Write all the values from a [Uint8List] into the buffer.
   void putUint8List(Uint8List list) {
-    _buffer.addAll(list);
+    _buffer!.addAll(list);
   }
 
   /// Write all the values from an [Int32List] into the buffer.
   void putInt32List(Int32List list) {
     _alignTo(4);
-    _buffer.addAll(list.buffer.asUint8List(list.offsetInBytes, 4 * list.length));
+    _buffer!.addAll(list.buffer.asUint8List(list.offsetInBytes, 4 * list.length));
   }
 
   /// Write all the values from an [Int64List] into the buffer.
   void putInt64List(Int64List list) {
     _alignTo(8);
-    _buffer.addAll(list.buffer.asUint8List(list.offsetInBytes, 8 * list.length));
+    _buffer!.addAll(list.buffer.asUint8List(list.offsetInBytes, 8 * list.length));
   }
 
   /// Write all the values from a [Float64List] into the buffer.
   void putFloat64List(Float64List list) {
     _alignTo(8);
-    _buffer.addAll(list.buffer.asUint8List(list.offsetInBytes, 8 * list.length));
+    _buffer!.addAll(list.buffer.asUint8List(list.offsetInBytes, 8 * list.length));
   }
 
   void _alignTo(int alignment) {
-    final int mod = _buffer.length % alignment;
+    final int mod = _buffer!.length % alignment;
     if (mod != 0) {
       for (int i = 0; i < alignment - mod; i++)
-        _buffer.add(0);
+        _buffer!.add(0);
     }
   }
 
   /// Finalize and return the written [ByteData].
   ByteData done() {
-    final ByteData result = _buffer.buffer.asByteData(0, _buffer.lengthInBytes);
+    final ByteData result = _buffer!.buffer.asByteData(0, _buffer!.lengthInBytes);
     _buffer = null;
     return result;
   }
@@ -124,35 +122,35 @@ class ReadBuffer {
   }
 
   /// Reads a Uint16 from the buffer.
-  int getUint16({Endian endian}) {
+  int getUint16({Endian? endian}) {
     final int value = data.getUint16(_position, endian ?? Endian.host);
     _position += 2;
     return value;
   }
 
   /// Reads a Uint32 from the buffer.
-  int getUint32({Endian endian}) {
+  int getUint32({Endian? endian}) {
     final int value = data.getUint32(_position, endian ?? Endian.host);
     _position += 4;
     return value;
   }
 
   /// Reads an Int32 from the buffer.
-  int getInt32({Endian endian}) {
+  int getInt32({Endian? endian}) {
     final int value = data.getInt32(_position, endian ?? Endian.host);
     _position += 4;
     return value;
   }
 
   /// Reads an Int64 from the buffer.
-  int getInt64({Endian endian}) {
+  int getInt64({Endian? endian}) {
     final int value = data.getInt64(_position, endian ?? Endian.host);
     _position += 8;
     return value;
   }
 
   /// Reads a Float64 from the buffer.
-  double getFloat64({Endian endian}) {
+  double getFloat64({Endian? endian}) {
     _alignTo(8);
     final double value = data.getFloat64(_position, endian ?? Endian.host);
     _position += 8;

packages/flutter/lib/src/foundation/stack_frame.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:ui' show hashValues;
 
 import 'package:meta/meta.dart';
@@ -29,16 +27,16 @@ class StackFrame {
   /// All parameters must not be null. The [className] may be the empty string
   /// if there is no class (e.g. for a top level library method).
   const StackFrame({
-    @required this.number,
-    @required this.column,
-    @required this.line,
-    @required this.packageScheme,
-    @required this.package,
-    @required this.packagePath,
+    required this.number,
+    required this.column,
+    required this.line,
+    required this.packageScheme,
+    required this.package,
+    required this.packagePath,
     this.className = '',
-    @required this.method,
+    required this.method,
     this.isConstructor = false,
-    @required this.source,
+    required this.source,
   })  : assert(number != null),
         assert(column != null),
         assert(line != null),
@@ -92,11 +90,11 @@ class StackFrame {
         // On the Web in non-debug builds the stack trace includes the exception
         // message that precedes the stack trace itself. fromStackTraceLine will
         // return null in that case. We will skip it here.
-        .skipWhile((StackFrame frame) => frame == null)
+        .whereType<StackFrame>()
         .toList();
   }
 
-  static StackFrame _parseWebFrame(String line) {
+  static StackFrame? _parseWebFrame(String line) {
     if (kDebugMode) {
       return _parseWebDebugFrame(line);
     } else {
@@ -111,15 +109,16 @@ class StackFrame {
     final RegExp parser = hasPackage
         ? RegExp(r'^(package.+) (\d+):(\d+)\s+(.+)$')
         : RegExp(r'^(.+) (\d+):(\d+)\s+(.+)$');
-    final Match match = parser.firstMatch(line);
+    Match? match = parser.firstMatch(line);
     assert(match != null, 'Expected $line to match $parser.');
+    match = match!;
 
     String package = '<unknown>';
     String packageScheme = '<unknown>';
     String packagePath = '<unknown>';
     if (hasPackage) {
       packageScheme = 'package';
-      final Uri packageUri = Uri.parse(match.group(1));
+      final Uri packageUri = Uri.parse(match.group(1)!);
       package = packageUri.pathSegments[0];
       packagePath = packageUri.path.replaceFirst(packageUri.pathSegments[0] + '/', '');
     }
@@ -129,10 +128,10 @@ class StackFrame {
       packageScheme: packageScheme,
       package: package,
       packagePath: packagePath,
-      line: int.parse(match.group(2)),
-      column: int.parse(match.group(3)),
+      line: int.parse(match.group(2)!),
+      column: int.parse(match.group(3)!),
       className: '<unknown>',
-      method: match.group(4),
+      method: match.group(4)!,
       source: line,
     );
   }
@@ -145,8 +144,8 @@ class StackFrame {
 
   // Parses `line` as a stack frame in profile and release Web builds. If not
   // recognized as a stack frame, returns null.
-  static StackFrame _parseWebNonDebugFrame(String line) {
-    final Match match = _webNonDebugFramePattern.firstMatch(line);
+  static StackFrame? _parseWebNonDebugFrame(String line) {
+    final Match? match = _webNonDebugFramePattern.firstMatch(line);
     if (match == null) {
       // On the Web in non-debug builds the stack trace includes the exception
       // message that precedes the stack trace itself. Example:
@@ -162,7 +161,7 @@ class StackFrame {
       return null;
     }
 
-    final List<String> classAndMethod = match.group(1).split('.');
+    final List<String> classAndMethod = match.group(1)!.split('.');
     final String className = classAndMethod.length > 1 ? classAndMethod.first : '<unknown>';
     final String method = classAndMethod.length > 1
       ? classAndMethod.skip(1).join('.')
@@ -182,7 +181,7 @@ class StackFrame {
   }
 
   /// Parses a single [StackFrame] from a single line of a [StackTrace].
-  static StackFrame fromStackTraceLine(String line) {
+  static StackFrame? fromStackTraceLine(String line) {
     assert(line != null);
     if (line == '<asynchronous suspension>') {
       return asynchronousSuspension;
@@ -203,12 +202,13 @@ class StackFrame {
     }
 
     final RegExp parser = RegExp(r'^#(\d+) +(.+) \((.+?):?(\d+){0,1}:?(\d+){0,1}\)$');
-    final Match match = parser.firstMatch(line);
+    Match? match = parser.firstMatch(line);
     assert(match != null, 'Expected $line to match $parser.');
+    match = match!;
 
     bool isConstructor = false;
     String className = '';
-    String method = match.group(2).replaceAll('.<anonymous closure>', '');
+    String method = match.group(2)!.replaceAll('.<anonymous closure>', '');
     if (method.startsWith('new')) {
       className = method.split(' ')[1];
       method = '';
@@ -224,7 +224,7 @@ class StackFrame {
       method = parts[1];
     }
 
-    final Uri packageUri = Uri.parse(match.group(3));
+    final Uri packageUri = Uri.parse(match.group(3)!);
     String package = '<unknown>';
     String packagePath = packageUri.path;
     if (packageUri.scheme == 'dart' || packageUri.scheme == 'package') {
@@ -233,14 +233,14 @@ class StackFrame {
     }
 
     return StackFrame(
-      number: int.parse(match.group(1)),
+      number: int.parse(match.group(1)!),
       className: className,
       method: method,
       packageScheme: packageUri.scheme,
       package: package,
       packagePath: packagePath,
-      line: match.group(4) == null ? -1 : int.parse(match.group(4)),
-      column: match.group(5) == null ? -1 : int.parse(match.group(5)),
+      line: match.group(4) == null ? -1 : int.parse(match.group(4)!),
+      column: match.group(5) == null ? -1 : int.parse(match.group(5)!),
       isConstructor: isConstructor,
       source: line,
     );

packages/flutter/lib/src/foundation/synchronous_future.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';
 
 /// A [Future] whose [then] implementation calls the callback immediately.
@@ -36,18 +34,18 @@ class SynchronousFuture<T> implements Future<T> {
   }
 
   @override
-  Future<T> catchError(Function onError, { bool test(Object error) }) => Completer<T>().future;
+  Future<T> catchError(Function onError, { bool test(Object error)? }) => Completer<T>().future;
 
   @override
-  Future<E> then<E>(FutureOr<E> f(T value), { Function onError }) {
-    final dynamic result = f(_value);
-    if (result is Future<E>)
+  Future<R> then<R>(FutureOr<R> onValue(T value), { Function? onError }) {
+    final dynamic result = onValue(_value);
+    if (result is Future<R>)
       return result;
-    return SynchronousFuture<E>(result as E);
+    return SynchronousFuture<R>(result as R);
   }
 
   @override
-  Future<T> timeout(Duration timeLimit, { FutureOr<T> onTimeout() }) {
+  Future<T> timeout(Duration timeLimit, { FutureOr<T> onTimeout()? }) {
     return Future<T>.value(_value).timeout(timeLimit, onTimeout: onTimeout);
   }
 

packages/flutter/lib/src/foundation/unicode.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
-
 /// Constants for useful Unicode characters.
 ///
 /// Currently, these characters are all related to bidirectional text.

packages/flutter/test/widgets/texture_test.dart

@@ -2,6 +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:flutter_test/flutter_test.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';