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/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/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);
-      }
+      _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';