Reland "Nnbd widgets" (#65528)

* Reland "Nnbd widgets (#64672)"

This reverts commit 127e6790.

* remove non-nullable enforcement on AsyncSnapshot

* make data param non-nullable for AsyncSnapshot.withData

* make some Text params non-nullable

packages/flutter/lib/src/cupertino/text_field.dart

@@ -645,7 +645,7 @@ class CupertinoTextField extends StatefulWidget {
   }
 }
 
-class _CupertinoTextFieldState extends State<CupertinoTextField> with RestorationMixin, AutomaticKeepAliveClientMixin implements TextSelectionGestureDetectorBuilderDelegate {
+class _CupertinoTextFieldState extends State<CupertinoTextField> with RestorationMixin, AutomaticKeepAliveClientMixin<CupertinoTextField> implements TextSelectionGestureDetectorBuilderDelegate {
   final GlobalKey _clearGlobalKey = GlobalKey();
 
   RestorableTextEditingController _controller;

packages/flutter/lib/src/rendering/sliver_fixed_extent_list.dart

@@ -196,28 +196,11 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
     if (firstChild == null) {
       if (!addInitialChild(index: firstIndex, layoutOffset: indexToLayoutOffset(itemExtent, firstIndex))) {
         // There are either no children, or we are past the end of all our children.
-        // If it is the latter, we will need to find the first available child.
         double max;
-        if (childManager.childCount != null) {
-          max = computeMaxScrollOffset(constraints, itemExtent);
-          // TODO(ianh): null-aware flow analysis flags the next two
-          // branches as entirely dead code, and it's hard to argue with
-          // its logic.
-        } else if (firstIndex <= 0) { // ignore: dead_code
+        if (firstIndex <= 0) {
           max = 0.0;
         } else {
-          // We will have to find it manually.
-          int possibleFirstIndex = firstIndex - 1;
-          while (
-            possibleFirstIndex > 0 &&
-            !addInitialChild(
-              index: possibleFirstIndex,
-              layoutOffset: indexToLayoutOffset(itemExtent, possibleFirstIndex),
-            )
-          ) {
-            possibleFirstIndex -= 1;
-          }
-          max = (possibleFirstIndex + 1) * itemExtent;
+          max = computeMaxScrollOffset(constraints, itemExtent);
         }
         geometry = SliverGeometry(
           scrollExtent: max,

packages/flutter/lib/src/rendering/viewport_offset.dart

@@ -94,6 +94,9 @@ abstract class ViewportOffset extends ChangeNotifier {
   /// the value changes due to [correctBy]).
   double get pixels;
 
+  /// Whether the [pixels] property is available.
+  bool get hasPixels;
+
   /// Called when the viewport's extents are established.
   ///
   /// The argument is the dimension of the [RenderViewport] in the main axis
@@ -245,8 +248,10 @@ abstract class ViewportOffset extends ChangeNotifier {
   /// `super.debugFillDescription(description)`.
   @mustCallSuper
   void debugFillDescription(List<String> description) {
+    if (hasPixels) {
       description.add('offset: ${pixels.toStringAsFixed(1)}');
     }
+  }
 }
 
 class _FixedViewportOffset extends ViewportOffset {
@@ -258,6 +263,9 @@ class _FixedViewportOffset extends ViewportOffset {
   @override
   double get pixels => _pixels;
 
+  @override
+  bool get hasPixels => true;
+
   @override
   bool applyViewportDimension(double viewportDimension) => true;
 

packages/flutter/lib/src/widgets/animated_cross_fade.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:flutter/rendering.dart';
 
 import 'animated_size.dart';
@@ -120,15 +118,15 @@ class AnimatedCrossFade extends StatefulWidget {
   ///
   /// All the arguments other than [key] must be non-null.
   const AnimatedCrossFade({
-    Key key,
-    @required this.firstChild,
-    @required this.secondChild,
+    Key? key,
+    required this.firstChild,
+    required this.secondChild,
     this.firstCurve = Curves.linear,
     this.secondCurve = Curves.linear,
     this.sizeCurve = Curves.linear,
     this.alignment = Alignment.topCenter,
-    @required this.crossFadeState,
-    @required this.duration,
+    required this.crossFadeState,
+    required this.duration,
     this.reverseDuration,
     this.layoutBuilder = defaultLayoutBuilder,
   }) : assert(firstChild != null),
@@ -163,7 +161,7 @@ class AnimatedCrossFade extends StatefulWidget {
   /// The duration of the whole orchestrated animation when running in reverse.
   ///
   /// If not supplied, this defaults to [duration].
-  final Duration reverseDuration;
+  final Duration? reverseDuration;
 
   /// The fade curve of the first child.
   ///
@@ -249,9 +247,9 @@ class AnimatedCrossFade extends StatefulWidget {
 }
 
 class _AnimatedCrossFadeState extends State<AnimatedCrossFade> with TickerProviderStateMixin {
-  AnimationController _controller;
-  Animation<double> _firstAnimation;
-  Animation<double> _secondAnimation;
+  AnimationController? _controller;
+  late Animation<double> _firstAnimation;
+  late Animation<double> _secondAnimation;
 
   @override
   void initState() {
@@ -262,10 +260,10 @@ class _AnimatedCrossFadeState extends State<AnimatedCrossFade> with TickerProvid
       vsync: this,
     );
     if (widget.crossFadeState == CrossFadeState.showSecond)
-      _controller.value = 1.0;
+      _controller!.value = 1.0;
     _firstAnimation = _initAnimation(widget.firstCurve, true);
     _secondAnimation = _initAnimation(widget.secondCurve, false);
-    _controller.addStatusListener((AnimationStatus status) {
+    _controller!.addStatusListener((AnimationStatus status) {
       setState(() {
         // Trigger a rebuild because it depends on _isTransitioning, which
         // changes its value together with animation status.
@@ -274,7 +272,7 @@ class _AnimatedCrossFadeState extends State<AnimatedCrossFade> with TickerProvid
   }
 
   Animation<double> _initAnimation(Curve curve, bool inverted) {
-    Animation<double> result = _controller.drive(CurveTween(curve: curve));
+    Animation<double> result = _controller!.drive(CurveTween(curve: curve));
     if (inverted)
       result = result.drive(Tween<double>(begin: 1.0, end: 0.0));
     return result;
@@ -282,7 +280,7 @@ class _AnimatedCrossFadeState extends State<AnimatedCrossFade> with TickerProvid
 
   @override
   void dispose() {
-    _controller.dispose();
+    _controller!.dispose();
     super.dispose();
   }
 
@@ -290,9 +288,9 @@ class _AnimatedCrossFadeState extends State<AnimatedCrossFade> with TickerProvid
   void didUpdateWidget(AnimatedCrossFade oldWidget) {
     super.didUpdateWidget(oldWidget);
     if (widget.duration != oldWidget.duration)
-      _controller.duration = widget.duration;
+      _controller!.duration = widget.duration;
     if (widget.reverseDuration != oldWidget.reverseDuration)
-      _controller.reverseDuration = widget.reverseDuration;
+      _controller!.reverseDuration = widget.reverseDuration;
     if (widget.firstCurve != oldWidget.firstCurve)
       _firstAnimation = _initAnimation(widget.firstCurve, true);
     if (widget.secondCurve != oldWidget.secondCurve)
@@ -300,24 +298,24 @@ class _AnimatedCrossFadeState extends State<AnimatedCrossFade> with TickerProvid
     if (widget.crossFadeState != oldWidget.crossFadeState) {
       switch (widget.crossFadeState) {
         case CrossFadeState.showFirst:
-          _controller.reverse();
+          _controller!.reverse();
           break;
         case CrossFadeState.showSecond:
-          _controller.forward();
+          _controller!.forward();
           break;
       }
     }
   }
 
   /// Whether we're in the middle of cross-fading this frame.
-  bool get _isTransitioning => _controller.status == AnimationStatus.forward || _controller.status == AnimationStatus.reverse;
+  bool get _isTransitioning => _controller!.status == AnimationStatus.forward || _controller!.status == AnimationStatus.reverse;
 
   @override
   Widget build(BuildContext context) {
     const Key kFirstChildKey = ValueKey<CrossFadeState>(CrossFadeState.showFirst);
     const Key kSecondChildKey = ValueKey<CrossFadeState>(CrossFadeState.showSecond);
-    final bool transitioningForwards = _controller.status == AnimationStatus.completed ||
-                                       _controller.status == AnimationStatus.forward;
+    final bool transitioningForwards = _controller!.status == AnimationStatus.completed ||
+                                       _controller!.status == AnimationStatus.forward;
     Key topKey;
     Widget topChild;
     Animation<double> topAnimation;

packages/flutter/lib/src/widgets/animated_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 'package:flutter/animation.dart';
 import 'package:flutter/foundation.dart';
 
@@ -32,8 +30,8 @@ class _ActiveItem implements Comparable<_ActiveItem> {
     : controller = null,
       removedItemBuilder = null;
 
-  final AnimationController controller;
-  final AnimatedListRemovedItemBuilder removedItemBuilder;
+  final AnimationController? controller;
+  final AnimatedListRemovedItemBuilder? removedItemBuilder;
   int itemIndex;
 
   @override
@@ -271,8 +269,8 @@ class AnimatedList extends StatefulWidget {
   /// Creates a scrolling container that animates items when they are inserted
   /// or removed.
   const AnimatedList({
-    Key key,
-    @required this.itemBuilder,
+    Key? key,
+    required this.itemBuilder,
     this.initialItemCount = 0,
     this.scrollDirection = Axis.vertical,
     this.reverse = false,
@@ -339,7 +337,7 @@ class AnimatedList extends StatefulWidget {
   /// [ScrollController.keepScrollOffset]). It can be used to read the current
   /// scroll position (see [ScrollController.offset]), or change it (see
   /// [ScrollController.animateTo]).
-  final ScrollController controller;
+  final ScrollController? controller;
 
   /// Whether this is the primary scroll view associated with the parent
   /// [PrimaryScrollController].
@@ -349,7 +347,7 @@ class AnimatedList extends StatefulWidget {
   ///
   /// Defaults to true when [scrollDirection] is [Axis.vertical] and
   /// [controller] is null.
-  final bool primary;
+  final bool? primary;
 
   /// How the scroll view should respond to user input.
   ///
@@ -357,7 +355,7 @@ class AnimatedList extends StatefulWidget {
   /// user stops dragging the scroll view.
   ///
   /// Defaults to matching platform conventions.
-  final ScrollPhysics physics;
+  final ScrollPhysics? physics;
 
   /// Whether the extent of the scroll view in the [scrollDirection] should be
   /// determined by the contents being viewed.
@@ -376,7 +374,7 @@ class AnimatedList extends StatefulWidget {
   final bool shrinkWrap;
 
   /// The amount of space by which to inset the children.
-  final EdgeInsetsGeometry padding;
+  final EdgeInsetsGeometry? padding;
 
   /// The state from the closest instance of this class that encloses the given
   /// context.
@@ -387,10 +385,10 @@ class AnimatedList extends StatefulWidget {
   /// ```dart
   /// AnimatedListState animatedList = AnimatedList.of(context);
   /// ```
-  static AnimatedListState of(BuildContext context, { bool nullOk = false }) {
+  static AnimatedListState? of(BuildContext context, { bool nullOk = false }) {
     assert(context != null);
     assert(nullOk != null);
-    final AnimatedListState result = context.findAncestorStateOfType<AnimatedListState>();
+    final AnimatedListState? result = context.findAncestorStateOfType<AnimatedListState>();
     if (nullOk || result != null)
       return result;
     throw FlutterError.fromParts(<DiagnosticsNode>[
@@ -444,7 +442,7 @@ class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixi
   /// it increases the length of the list by one and shifts all items at or
   /// after [index] towards the end of the list.
   void insertItem(int index, { Duration duration = _kDuration }) {
-    _sliverAnimatedListKey.currentState.insertItem(index, duration: duration);
+    _sliverAnimatedListKey.currentState!.insertItem(index, duration: duration);
   }
 
   /// Remove the item at [index] and start an animation that will be passed
@@ -459,7 +457,7 @@ class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixi
   /// it decreases the length of the list by one and shifts all items at or
   /// before [index] towards the beginning of the list.
   void removeItem(int index, AnimatedListRemovedItemBuilder builder, { Duration duration = _kDuration }) {
-    _sliverAnimatedListKey.currentState.removeItem(index, builder, duration: duration);
+    _sliverAnimatedListKey.currentState!.removeItem(index, builder, duration: duration);
   }
 
   @override
@@ -736,8 +734,8 @@ class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixi
 class SliverAnimatedList extends StatefulWidget {
   /// Creates a sliver that animates items when they are inserted or removed.
   const SliverAnimatedList({
-    Key key,
-    @required this.itemBuilder,
+    Key? key,
+    required this.itemBuilder,
     this.initialItemCount = 0,
   }) : assert(itemBuilder != null),
        assert(initialItemCount != null && initialItemCount >= 0),
@@ -773,10 +771,10 @@ class SliverAnimatedList extends StatefulWidget {
   /// ```dart
   /// SliverAnimatedListState animatedList = SliverAnimatedList.of(context);
   /// ```
-  static SliverAnimatedListState of(BuildContext context, {bool nullOk = false}) {
+  static SliverAnimatedListState? of(BuildContext context, {bool nullOk = false}) {
     assert(context != null);
     assert(nullOk != null);
-    final SliverAnimatedListState result = context.findAncestorStateOfType<SliverAnimatedListState>();
+    final SliverAnimatedListState? result = context.findAncestorStateOfType<SliverAnimatedListState>();
     if (nullOk || result != null)
       return result;
     throw FlutterError(
@@ -831,17 +829,17 @@ class SliverAnimatedListState extends State<SliverAnimatedList> with TickerProvi
   @override
   void dispose() {
     for (final _ActiveItem item in _incomingItems.followedBy(_outgoingItems)) {
-      item.controller.dispose();
+      item.controller!.dispose();
     }
     super.dispose();
   }
 
-  _ActiveItem _removeActiveItemAt(List<_ActiveItem> items, int itemIndex) {
+  _ActiveItem? _removeActiveItemAt(List<_ActiveItem> items, int itemIndex) {
     final int i = binarySearch(items, _ActiveItem.index(itemIndex));
     return i == -1 ? null : items.removeAt(i);
   }
 
-  _ActiveItem _activeItemAt(List<_ActiveItem> items, int itemIndex) {
+  _ActiveItem? _activeItemAt(List<_ActiveItem> items, int itemIndex) {
     final int i = binarySearch(items, _ActiveItem.index(itemIndex));
     return i == -1 ? null : items[i];
   }
@@ -919,7 +917,7 @@ class SliverAnimatedListState extends State<SliverAnimatedList> with TickerProvi
     });
 
     controller.forward().then<void>((_) {
-      _removeActiveItemAt(_incomingItems, incomingItem.itemIndex).controller.dispose();
+      _removeActiveItemAt(_incomingItems, incomingItem.itemIndex)!.controller!.dispose();
     });
   }
 
@@ -943,7 +941,7 @@ class SliverAnimatedListState extends State<SliverAnimatedList> with TickerProvi
     assert(itemIndex >= 0 && itemIndex < _itemsCount);
     assert(_activeItemAt(_outgoingItems, itemIndex) == null);
 
-    final _ActiveItem incomingItem = _removeActiveItemAt(_incomingItems, itemIndex);
+    final _ActiveItem? incomingItem = _removeActiveItemAt(_incomingItems, itemIndex);
     final AnimationController controller = incomingItem?.controller
       ?? AnimationController(duration: duration, value: 1.0, vsync: this);
     final _ActiveItem outgoingItem = _ActiveItem.outgoing(controller, itemIndex, builder);
@@ -954,7 +952,7 @@ class SliverAnimatedListState extends State<SliverAnimatedList> with TickerProvi
     });
 
     controller.reverse().then<void>((void value) {
-      _removeActiveItemAt(_outgoingItems, outgoingItem.itemIndex).controller.dispose();
+      _removeActiveItemAt(_outgoingItems, outgoingItem.itemIndex)!.controller!.dispose();
 
       // Decrement the incoming and outgoing item indices to account
       // for the removal.
@@ -972,15 +970,15 @@ class SliverAnimatedListState extends State<SliverAnimatedList> with TickerProvi
   }
 
   Widget _itemBuilder(BuildContext context, int itemIndex) {
-    final _ActiveItem outgoingItem = _activeItemAt(_outgoingItems, itemIndex);
+    final _ActiveItem? outgoingItem = _activeItemAt(_outgoingItems, itemIndex);
     if (outgoingItem != null) {
-      return outgoingItem.removedItemBuilder(
+      return outgoingItem.removedItemBuilder!(
         context,
-        outgoingItem.controller.view,
+        outgoingItem.controller!.view,
       );
     }
 
-    final _ActiveItem incomingItem = _activeItemAt(_incomingItems, itemIndex);
+    final _ActiveItem? incomingItem = _activeItemAt(_incomingItems, itemIndex);
     final Animation<double> animation = incomingItem?.controller?.view ?? kAlwaysCompleteAnimation;
     return widget.itemBuilder(
       context,

packages/flutter/lib/src/widgets/animated_size.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:flutter/rendering.dart';
 import 'package:flutter/scheduler.dart';
 
@@ -56,13 +54,13 @@ class AnimatedSize extends SingleChildRenderObjectWidget {
   ///
   /// The [curve] and [duration] arguments must not be null.
   const AnimatedSize({
-    Key key,
-    Widget child,
+    Key? key,
+    Widget? child,
     this.alignment = Alignment.center,
     this.curve = Curves.linear,
-    @required this.duration,
+    required this.duration,
     this.reverseDuration,
-    @required this.vsync,
+    required this.vsync,
   }) : super(key: key, child: child);
 
   /// The alignment of the child within the parent when the parent is not yet
@@ -98,7 +96,7 @@ class AnimatedSize extends SingleChildRenderObjectWidget {
   /// size when going in reverse.
   ///
   /// If not specified, defaults to [duration].
-  final Duration reverseDuration;
+  final Duration? reverseDuration;
 
   /// The [TickerProvider] for this widget.
   final TickerProvider vsync;

packages/flutter/lib/src/widgets/animated_switcher.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:flutter/animation.dart';
 import 'package:flutter/foundation.dart';
 
@@ -18,10 +16,10 @@ import 'transitions.dart';
 // to expose to the public API (like the controller).
 class _ChildEntry {
   _ChildEntry({
-    @required this.controller,
-    @required this.animation,
-    @required this.transition,
-    @required this.widgetChild,
+    required this.controller,
+    required this.animation,
+    required this.transition,
+    required this.widgetChild,
   }) : assert(animation != null),
        assert(transition != null),
        assert(controller != null);
@@ -63,7 +61,7 @@ typedef AnimatedSwitcherTransitionBuilder = Widget Function(Widget child, Animat
 /// The `previousChildren` list is an unmodifiable list, sorted with the oldest
 /// at the beginning and the newest at the end. It does not include the
 /// `currentChild`.
-typedef AnimatedSwitcherLayoutBuilder = Widget Function(Widget currentChild, List<Widget> previousChildren);
+typedef AnimatedSwitcherLayoutBuilder = Widget Function(Widget? currentChild, List<Widget> previousChildren);
 
 /// A widget that by default does a cross-fade between a new widget and the
 /// widget previously set on the [AnimatedSwitcher] as a child.
@@ -148,9 +146,9 @@ class AnimatedSwitcher extends StatefulWidget {
   /// The [duration], [transitionBuilder], [layoutBuilder], [switchInCurve], and
   /// [switchOutCurve] parameters must not be null.
   const AnimatedSwitcher({
-    Key key,
+    Key? key,
     this.child,
-    @required this.duration,
+    required this.duration,
     this.reverseDuration,
     this.switchInCurve = Curves.linear,
     this.switchOutCurve = Curves.linear,
@@ -174,7 +172,7 @@ class AnimatedSwitcher extends StatefulWidget {
   /// (see [Widget.canUpdate]).
   ///
   /// To change the kind of transition used, see [transitionBuilder].
-  final Widget child;
+  final Widget? child;
 
   /// The duration of the transition from the old [child] value to the new one.
   ///
@@ -191,7 +189,7 @@ class AnimatedSwitcher extends StatefulWidget {
   /// transitions already in progress.
   ///
   /// If not set, then the value of [duration] is used by default.
-  final Duration reverseDuration;
+  final Duration? reverseDuration;
 
   /// The animation curve to use when transitioning in a new [child].
   ///
@@ -274,7 +272,7 @@ class AnimatedSwitcher extends StatefulWidget {
   /// each other.
   ///
   /// This is an [AnimatedSwitcherLayoutBuilder] function.
-  static Widget defaultLayoutBuilder(Widget currentChild, List<Widget> previousChildren) {
+  static Widget defaultLayoutBuilder(Widget? currentChild, List<Widget> previousChildren) {
     return Stack(
       children: <Widget>[
         ...previousChildren,
@@ -293,9 +291,9 @@ class AnimatedSwitcher extends StatefulWidget {
 }
 
 class _AnimatedSwitcherState extends State<AnimatedSwitcher> with TickerProviderStateMixin {
-  _ChildEntry _currentEntry;
+  _ChildEntry? _currentEntry;
   final Set<_ChildEntry> _outgoingEntries = <_ChildEntry>{};
-  List<Widget> _outgoingWidgets = const <Widget>[];
+  List<Widget>? _outgoingWidgets = const <Widget>[];
   int _childNumber = 0;
 
   @override
@@ -313,37 +311,37 @@ class _AnimatedSwitcherState extends State<AnimatedSwitcher> with TickerProvider
     if (widget.transitionBuilder != oldWidget.transitionBuilder) {
       _outgoingEntries.forEach(_updateTransitionForEntry);
       if (_currentEntry != null)
-        _updateTransitionForEntry(_currentEntry);
+        _updateTransitionForEntry(_currentEntry!);
       _markChildWidgetCacheAsDirty();
     }
 
     final bool hasNewChild = widget.child != null;
     final bool hasOldChild = _currentEntry != null;
     if (hasNewChild != hasOldChild ||
-        hasNewChild && !Widget.canUpdate(widget.child, _currentEntry.widgetChild)) {
+        hasNewChild && !Widget.canUpdate(widget.child!, _currentEntry!.widgetChild)) {
       // Child has changed, fade current entry out and add new entry.
       _childNumber += 1;
       _addEntryForNewChild(animate: true);
     } else if (_currentEntry != null) {
       assert(hasOldChild && hasNewChild);
-      assert(Widget.canUpdate(widget.child, _currentEntry.widgetChild));
+      assert(Widget.canUpdate(widget.child!, _currentEntry!.widgetChild));
       // Child has been updated. Make sure we update the child widget and
       // transition in _currentEntry even though we're not going to start a new
       // animation, but keep the key from the previous transition so that we
       // update the transition instead of replacing it.
-      _currentEntry.widgetChild = widget.child;
-      _updateTransitionForEntry(_currentEntry); // uses entry.widgetChild
+      _currentEntry!.widgetChild = widget.child!;
+      _updateTransitionForEntry(_currentEntry!); // uses entry.widgetChild
       _markChildWidgetCacheAsDirty();
     }
   }
 
-  void _addEntryForNewChild({ @required bool animate }) {
+  void _addEntryForNewChild({ required bool animate }) {
     assert(animate || _currentEntry == null);
     if (_currentEntry != null) {
       assert(animate);
       assert(!_outgoingEntries.contains(_currentEntry));
-      _outgoingEntries.add(_currentEntry);
-      _currentEntry.controller.reverse();
+      _outgoingEntries.add(_currentEntry!);
+      _currentEntry!.controller.reverse();
       _markChildWidgetCacheAsDirty();
       _currentEntry = null;
     }
@@ -360,7 +358,7 @@ class _AnimatedSwitcherState extends State<AnimatedSwitcher> with TickerProvider
       reverseCurve: widget.switchOutCurve,
     );
     _currentEntry = _newEntry(
-      child: widget.child,
+      child: widget.child!,
       controller: controller,
       animation: animation,
       builder: widget.transitionBuilder,
@@ -374,10 +372,10 @@ class _AnimatedSwitcherState extends State<AnimatedSwitcher> with TickerProvider
   }
 
   _ChildEntry _newEntry({
-    @required Widget child,
-    @required AnimatedSwitcherTransitionBuilder builder,
-    @required AnimationController controller,
-    @required Animation<double> animation,
+    required Widget child,
+    required AnimatedSwitcherTransitionBuilder builder,
+    required AnimationController controller,
+    required Animation<double> animation,
   }) {
     final _ChildEntry entry = _ChildEntry(
       widgetChild: child,
@@ -414,14 +412,14 @@ class _AnimatedSwitcherState extends State<AnimatedSwitcher> with TickerProvider
     _outgoingWidgets ??= List<Widget>.unmodifiable(
       _outgoingEntries.map<Widget>((_ChildEntry entry) => entry.transition),
     );
-    assert(_outgoingEntries.length == _outgoingWidgets.length);
-    assert(_outgoingEntries.isEmpty || _outgoingEntries.last.transition == _outgoingWidgets.last);
+    assert(_outgoingEntries.length == _outgoingWidgets!.length);
+    assert(_outgoingEntries.isEmpty || _outgoingEntries.last.transition == _outgoingWidgets!.last);
   }
 
   @override
   void dispose() {
     if (_currentEntry != null)
-      _currentEntry.controller.dispose();
+      _currentEntry!.controller.dispose();
     for (final _ChildEntry entry in _outgoingEntries)
       entry.controller.dispose();
     super.dispose();
@@ -430,6 +428,6 @@ class _AnimatedSwitcherState extends State<AnimatedSwitcher> with TickerProvider
   @override
   Widget build(BuildContext context) {
     _rebuildOutgoingWidgetsIfNeeded();
-    return widget.layoutBuilder(_currentEntry?.transition, _outgoingWidgets);
+    return widget.layoutBuilder(_currentEntry?.transition, _outgoingWidgets!);
   }
 }

packages/flutter/lib/src/widgets/annotated_region.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:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 
@@ -15,7 +13,7 @@ import 'framework.dart';
 ///
 ///  * [Layer.find], for an example of how this value is retrieved.
 ///  * [AnnotatedRegionLayer], the layer pushed into the layer tree.
-class AnnotatedRegion<T> extends SingleChildRenderObjectWidget {
+class AnnotatedRegion<T extends Object> extends SingleChildRenderObjectWidget {
   /// Creates a new annotated region to insert [value] into the layer tree.
   ///
   /// Neither [child] nor [value] may be null.
@@ -23,9 +21,9 @@ class AnnotatedRegion<T> extends SingleChildRenderObjectWidget {
   /// [sized] defaults to true and controls whether the annotated region will
   /// clip its child.
   const AnnotatedRegion({
-    Key key,
-    @required Widget child,
-    @required this.value,
+    Key? key,
+    required Widget child,
+    required this.value,
     this.sized = true,
   }) : assert(value != null),
        assert(child != null),

packages/flutter/lib/src/widgets/async.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
-
 /// Widgets that handle interaction with asynchronous computations.
 ///
 /// Asynchronous computations are represented by [Future]s and [Stream]s.
@@ -50,13 +48,13 @@ import 'framework.dart';
 ///    recent interaction is needed for widget building.
 abstract class StreamBuilderBase<T, S> extends StatefulWidget {
   /// Creates a [StreamBuilderBase] connected to the specified [stream].
-  const StreamBuilderBase({ Key key, this.stream }) : super(key: key);
+  const StreamBuilderBase({ Key? key, this.stream }) : super(key: key);
 
   /// The asynchronous computation to which this builder is currently connected,
   /// possibly null. When changed, the current summary is updated using
   /// [afterDisconnected], if the previous stream was not null, followed by
   /// [afterConnected], if the new stream is not null.
-  final Stream<T> stream;
+  final Stream<T>? stream;
 
   /// Returns the initial summary of stream interaction, typically representing
   /// the fact that no interaction has happened at all.
@@ -103,8 +101,8 @@ abstract class StreamBuilderBase<T, S> extends StatefulWidget {
 
 /// State for [StreamBuilderBase].
 class _StreamBuilderBaseState<T, S> extends State<StreamBuilderBase<T, S>> {
-  StreamSubscription<T> _subscription;
-  S _summary;
+  StreamSubscription<T>? _subscription; // ignore: cancel_subscriptions
+  late S _summary;
 
   @override
   void initState() {
@@ -136,7 +134,7 @@ class _StreamBuilderBaseState<T, S> extends State<StreamBuilderBase<T, S>> {
 
   void _subscribe() {
     if (widget.stream != null) {
-      _subscription = widget.stream.listen((T data) {
+      _subscription = widget.stream!.listen((T data) {
         setState(() {
           _summary = widget.afterData(_summary, data);
         });
@@ -155,7 +153,7 @@ class _StreamBuilderBaseState<T, S> extends State<StreamBuilderBase<T, S>> {
 
   void _unsubscribe() {
     if (_subscription != null) {
-      _subscription.cancel();
+      _subscription!.cancel();
       _subscription = null;
     }
   }
@@ -203,6 +201,8 @@ enum ConnectionState {
 ///    with a [Stream].
 ///  * [FutureBuilder], which builds itself based on a snapshot from interacting
 ///    with a [Future].
+// TODO(a14n): the type parameter should be <T extends Object> to enforce that retrieved data must not be null.
+// But it breaks a lot of code in google3. See https://github.com/flutter/flutter/pull/64672#pullrequestreview-485199027
 @immutable
 class AsyncSnapshot<T> {
   /// Creates an [AsyncSnapshot] with the specified [connectionState],
@@ -218,7 +218,7 @@ class AsyncSnapshot<T> {
   const AsyncSnapshot.waiting() : this._(ConnectionState.waiting, null, null);
 
   /// Creates an [AsyncSnapshot] in the specified [state] and with the specified [data].
-  const AsyncSnapshot.withData(ConnectionState state, T data) : this._(state, data, null);
+  const AsyncSnapshot.withData(ConnectionState state, T data): this._(state, data, null);
 
   /// Creates an [AsyncSnapshot] in the specified [state] and with the specified [error].
   const AsyncSnapshot.withError(ConnectionState state, Object error) : this._(state, null, error);
@@ -235,7 +235,7 @@ class AsyncSnapshot<T> {
   /// If the asynchronous computation has never returned a value, this may be
   /// set to an initial data value specified by the relevant widget. See
   /// [FutureBuilder.initialData] and [StreamBuilder.initialData].
-  final T data;
+  final T? data;
 
   /// Returns latest data received, failing if there is no data.
   ///
@@ -243,9 +243,9 @@ class AsyncSnapshot<T> {
   /// nor [hasError].
   T get requireData {
     if (hasData)
-      return data;
+      return data!;
     if (hasError)
-      throw error;
+      throw error!;
     throw StateError('Snapshot has neither data nor error');
   }
 
@@ -254,7 +254,7 @@ class AsyncSnapshot<T> {
   /// If this is non-null, [hasError] will be true.
   ///
   /// If [data] is not null, this will be null.
-  final Object error;
+  final Object? error;
 
   /// Returns a snapshot like this one, but in the specified [state].
   ///
@@ -467,7 +467,7 @@ typedef AsyncWidgetBuilder<T> = Widget Function(BuildContext context, AsyncSnaps
 ///  * [StreamBuilderBase], which supports widget building based on a computation
 ///    that spans all interactions made with the stream.
 // TODO(ianh): remove unreachable code above once https://github.com/dart-lang/linter/issues/1139 is fixed
-class StreamBuilder<T> extends StreamBuilderBase<T, AsyncSnapshot<T>> {
+class StreamBuilder<T extends Object> extends StreamBuilderBase<T, AsyncSnapshot<T>> {
   /// Creates a new [StreamBuilder] that builds itself based on the latest
   /// snapshot of interaction with the specified [stream] and whose build
   /// strategy is given by [builder].
@@ -476,10 +476,10 @@ class StreamBuilder<T> extends StreamBuilderBase<T, AsyncSnapshot<T>> {
   ///
   /// The [builder] must not be null.
   const StreamBuilder({
-    Key key,
+    Key? key,
     this.initialData,
-    Stream<T> stream,
-    @required this.builder,
+    Stream<T>? stream,
+    required this.builder,
   }) : assert(builder != null),
        super(key: key, stream: stream);
 
@@ -497,10 +497,12 @@ class StreamBuilder<T> extends StreamBuilderBase<T, AsyncSnapshot<T>> {
   /// of whether a value is available on the stream: since streams are
   /// asynchronous, no events from the stream can be obtained before the initial
   /// build.
-  final T initialData;
+  final T? initialData;
 
   @override
-  AsyncSnapshot<T> initial() => AsyncSnapshot<T>.withData(ConnectionState.none, initialData);
+  AsyncSnapshot<T> initial() => initialData == null
+      ? AsyncSnapshot<T>.nothing()
+      : AsyncSnapshot<T>.withData(ConnectionState.none, initialData!);
 
   @override
   AsyncSnapshot<T> afterConnected(AsyncSnapshot<T> current) => current.inState(ConnectionState.waiting);
@@ -662,16 +664,16 @@ class StreamBuilder<T> extends StreamBuilderBase<T, AsyncSnapshot<T>> {
 /// ```
 /// {@end-tool}
 // TODO(ianh): remove unreachable code above once https://github.com/dart-lang/sdk/issues/35520 is fixed
-class FutureBuilder<T> extends StatefulWidget {
+class FutureBuilder<T extends Object> extends StatefulWidget {
   /// Creates a widget that builds itself based on the latest snapshot of
   /// interaction with a [Future].
   ///
   /// The [builder] must not be null.
   const FutureBuilder({
-    Key key,
+    Key? key,
     this.future,
     this.initialData,
-    @required this.builder,
+    required this.builder,
   }) : assert(builder != null),
        super(key: key);
 
@@ -680,7 +682,7 @@ class FutureBuilder<T> extends StatefulWidget {
   ///
   /// If no future has yet completed, including in the case where [future] is
   /// null, the data provided to the [builder] will be set to [initialData].
-  final Future<T> future;
+  final Future<T>? future;
 
   /// The build strategy currently used by this builder.
   ///
@@ -714,24 +716,26 @@ class FutureBuilder<T> extends StatefulWidget {
   /// provided to the [builder] will become null, regardless of [initialData].
   /// (The error itself will be available in [AsyncSnapshot.error], and
   /// [AsyncSnapshot.hasError] will be true.)
-  final T initialData;
+  final T? initialData;
 
   @override
   State<FutureBuilder<T>> createState() => _FutureBuilderState<T>();
 }
 
 /// State for [FutureBuilder].
-class _FutureBuilderState<T> extends State<FutureBuilder<T>> {
+class _FutureBuilderState<T extends Object> extends State<FutureBuilder<T>> {
   /// An object that identifies the currently active callbacks. Used to avoid
   /// calling setState from stale callbacks, e.g. after disposal of this state,
   /// or after widget reconfiguration to a new Future.
-  Object _activeCallbackIdentity;
-  AsyncSnapshot<T> _snapshot;
+  Object? _activeCallbackIdentity;
+  late AsyncSnapshot<T> _snapshot;
 
   @override
   void initState() {
     super.initState();
-    _snapshot = AsyncSnapshot<T>.withData(ConnectionState.none, widget.initialData);
+    _snapshot = widget.initialData == null
+        ? AsyncSnapshot<T>.nothing()
+        : AsyncSnapshot<T>.withData(ConnectionState.none, widget.initialData!);
     _subscribe();
   }
 
@@ -760,7 +764,7 @@ class _FutureBuilderState<T> extends State<FutureBuilder<T>> {
     if (widget.future != null) {
       final Object callbackIdentity = Object();
       _activeCallbackIdentity = callbackIdentity;
-      widget.future.then<void>((T data) {
+      widget.future!.then<void>((T data) {
         if (_activeCallbackIdentity == callbackIdentity) {
           setState(() {
             _snapshot = AsyncSnapshot<T>.withData(ConnectionState.done, data);

packages/flutter/lib/src/widgets/autofill.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:flutter/services.dart';
 import 'framework.dart';
 
@@ -152,8 +150,8 @@ class AutofillGroup extends StatefulWidget {
   ///
   /// The [child] argument must not be null.
   const AutofillGroup({
-    Key key,
-    @required this.child,
+    Key? key,
+    required this.child,
     this.onDisposeAction = AutofillContextAction.commit,
   }) : assert(child != null),
        super(key: key);
@@ -166,8 +164,8 @@ class AutofillGroup extends StatefulWidget {
   ///
   /// * [EditableTextState], where this method is used to retrieve the closest
   ///   [AutofillGroupState].
-  static AutofillGroupState of(BuildContext context) {
-    final _AutofillScope scope = context.dependOnInheritedWidgetOfExactType<_AutofillScope>();
+  static AutofillGroupState? of(BuildContext context) {
+    final _AutofillScope? scope = context.dependOnInheritedWidgetOfExactType<_AutofillScope>();
     return scope?._scope;
   }
 
@@ -214,12 +212,12 @@ class AutofillGroupState extends State<AutofillGroup> with AutofillScopeMixin {
   bool _isTopmostAutofillGroup = false;
 
   @override
-  AutofillClient getAutofillClient(String tag) => _clients[tag];
+  AutofillClient? getAutofillClient(String tag) => _clients[tag];
 
   @override
   Iterable<AutofillClient> get autofillClients {
     return _clients.values
-      .where((AutofillClient client) => client?.textInputConfiguration?.autofillConfiguration != null);
+      .where((AutofillClient client) => client.textInputConfiguration.autofillConfiguration != null);
   }
 
   /// Adds the [AutofillClient] to this [AutofillGroup].
@@ -289,15 +287,15 @@ class AutofillGroupState extends State<AutofillGroup> with AutofillScopeMixin {
 
 class _AutofillScope extends InheritedWidget {
   const _AutofillScope({
-    Key key,
-    Widget child,
-    AutofillGroupState autofillScopeState,
+    Key? key,
+    required Widget child,
+    AutofillGroupState? autofillScopeState,
   }) : _scope = autofillScopeState,
        super(key: key, child: child);
 
-  final AutofillGroupState _scope;
+  final AutofillGroupState? _scope;
 
-  AutofillGroup get client => _scope.widget;
+  AutofillGroup get client => _scope!.widget;
 
   @override
   bool updateShouldNotify(_AutofillScope old) => _scope != old._scope;

packages/flutter/lib/src/widgets/automatic_keep_alive.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:flutter/foundation.dart';
@@ -29,22 +27,22 @@ class AutomaticKeepAlive extends StatefulWidget {
   /// Creates a widget that listens to [KeepAliveNotification]s and maintains a
   /// [KeepAlive] widget appropriately.
   const AutomaticKeepAlive({
-    Key key,
+    Key? key,
     this.child,
   }) : super(key: key);
 
   /// The widget below this widget in the tree.
   ///
   /// {@macro flutter.widgets.child}
-  final Widget child;
+  final Widget? child;
 
   @override
   _AutomaticKeepAliveState createState() => _AutomaticKeepAliveState();
 }
 
 class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
-  Map<Listenable, VoidCallback> _handles;
-  Widget _child;
+  Map<Listenable, VoidCallback>? _handles;
+  Widget? _child;
   bool _keepingAlive = false;
 
   @override
@@ -62,15 +60,15 @@ class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
   void _updateChild() {
     _child = NotificationListener<KeepAliveNotification>(
       onNotification: _addClient,
-      child: widget.child,
+      child: widget.child!,
     );
   }
 
   @override
   void dispose() {
     if (_handles != null) {
-      for (final Listenable handle in _handles.keys)
-        handle.removeListener(_handles[handle]);
+      for (final Listenable handle in _handles!.keys)
+        handle.removeListener(_handles![handle]!);
     }
     super.dispose();
   }
@@ -78,12 +76,12 @@ class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
   bool _addClient(KeepAliveNotification notification) {
     final Listenable handle = notification.handle;
     _handles ??= <Listenable, VoidCallback>{};
-    assert(!_handles.containsKey(handle));
-    _handles[handle] = _createCallback(handle);
-    handle.addListener(_handles[handle]);
+    assert(!_handles!.containsKey(handle));
+    _handles![handle] = _createCallback(handle);
+    handle.addListener(_handles![handle]!);
     if (!_keepingAlive) {
       _keepingAlive = true;
-      final ParentDataElement<KeepAliveParentDataMixin> childElement = _getChildElement();
+      final ParentDataElement<KeepAliveParentDataMixin>? childElement = _getChildElement();
       if (childElement != null) {
         // If the child already exists, update it synchronously.
         _updateParentDataOfChild(childElement);
@@ -91,13 +89,13 @@ class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
         // If the child doesn't exist yet, we got called during the very first
         // build of this subtree. Wait until the end of the frame to update
         // the child when the child is guaranteed to be present.
-        SchedulerBinding.instance.addPostFrameCallback((Duration timeStamp) {
+        SchedulerBinding.instance!.addPostFrameCallback((Duration timeStamp) {
           if (!mounted) {
             return;
           }
-          final ParentDataElement<KeepAliveParentDataMixin> childElement = _getChildElement();
+          final ParentDataElement<KeepAliveParentDataMixin>? childElement = _getChildElement();
           assert(childElement != null);
-          _updateParentDataOfChild(childElement);
+          _updateParentDataOfChild(childElement!);
         });
       }
     }
@@ -108,10 +106,10 @@ class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
   ///
   /// While this widget is guaranteed to have a child, this may return null if
   /// the first build of that child has not completed yet.
-  ParentDataElement<KeepAliveParentDataMixin> _getChildElement() {
+  ParentDataElement<KeepAliveParentDataMixin>? _getChildElement() {
     assert(mounted);
     final Element element = context as Element;
-    Element childElement;
+    Element? childElement;
     // We use Element.visitChildren rather than context.visitChildElements
     // because we might be called during build, and context.visitChildElements
     // verifies that it is not called during build. Element.visitChildren does
@@ -135,7 +133,7 @@ class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
       childElement = child;
     });
     assert(childElement == null || childElement is ParentDataElement<KeepAliveParentDataMixin>);
-    return childElement as ParentDataElement<KeepAliveParentDataMixin>;
+    return childElement as ParentDataElement<KeepAliveParentDataMixin>?;
   }
 
   void _updateParentDataOfChild(ParentDataElement<KeepAliveParentDataMixin> childElement) {
@@ -155,9 +153,9 @@ class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
         }
         return true;
       }());
-      _handles.remove(handle);
-      if (_handles.isEmpty) {
-        if (SchedulerBinding.instance.schedulerPhase.index < SchedulerPhase.persistentCallbacks.index) {
+      _handles!.remove(handle);
+      if (_handles!.isEmpty) {
+        if (SchedulerBinding.instance!.schedulerPhase.index < SchedulerPhase.persistentCallbacks.index) {
           // Build/layout haven't started yet so let's just schedule this for
           // the next frame.
           setState(() { _keepingAlive = false; });
@@ -213,7 +211,7 @@ class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
           // avoid it if possible.
           _keepingAlive = false;
           scheduleMicrotask(() {
-            if (mounted && _handles.isEmpty) {
+            if (mounted && _handles!.isEmpty) {
               // If mounted is false, we went away as well, so there's nothing to do.
               // If _handles is no longer empty, then another client (or the same
               // client in a new place) registered itself before we had a chance to
@@ -233,7 +231,7 @@ class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
     assert(_child != null);
     return KeepAlive(
       keepAlive: _keepingAlive,
-      child: _child,
+      child: _child!,
     );
   }
 
@@ -246,7 +244,7 @@ class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
       'handles',
       _handles,
       description: _handles != null ?
-        '${_handles.length} active client${ _handles.length == 1 ? "" : "s" }' :
+        '${_handles!.length} active client${ _handles!.length == 1 ? "" : "s" }' :
         null,
       ifNull: 'no notifications ever received',
     ));
@@ -330,8 +328,7 @@ class KeepAliveHandle extends ChangeNotifier {
 /// with [State] subclasses.
 ///
 /// Subclasses must implement [wantKeepAlive], and their [build] methods must
-/// call `super.build` (the return value will always return null, and should be
-/// ignored).
+/// call `super.build` (though the return value should be ignored).
 ///
 /// Then, whenever [wantKeepAlive]'s value changes (or might change), the
 /// subclass should call [updateKeepAlive].
@@ -345,16 +342,16 @@ class KeepAliveHandle extends ChangeNotifier {
 ///  * [KeepAliveNotification], the notifications sent by this mixin.
 @optionalTypeArgs
 mixin AutomaticKeepAliveClientMixin<T extends StatefulWidget> on State<T> {
-  KeepAliveHandle _keepAliveHandle;
+  KeepAliveHandle? _keepAliveHandle;
 
   void _ensureKeepAlive() {
     assert(_keepAliveHandle == null);
     _keepAliveHandle = KeepAliveHandle();
-    KeepAliveNotification(_keepAliveHandle).dispatch(context);
+    KeepAliveNotification(_keepAliveHandle!).dispatch(context);
   }
 
   void _releaseKeepAlive() {
-    _keepAliveHandle.release();
+    _keepAliveHandle!.release();
     _keepAliveHandle = null;
   }
 
@@ -397,6 +394,18 @@ mixin AutomaticKeepAliveClientMixin<T extends StatefulWidget> on State<T> {
   Widget build(BuildContext context) {
     if (wantKeepAlive && _keepAliveHandle == null)
       _ensureKeepAlive();
-    return null;
+    return const _NullWidget();
+  }
+}
+
+class _NullWidget extends StatelessWidget {
+  const _NullWidget();
+
+  @override
+  Widget build(BuildContext context) {
+    throw FlutterError(
+      'Widgets that mix AutomaticKeepAliveClientMixin into their State must '
+      'call super.build() but must ignore the return value of the superclass.'
+    );
   }
 }

packages/flutter/lib/src/widgets/banner.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:flutter/foundation.dart';
@@ -61,10 +59,10 @@ class BannerPainter extends CustomPainter {
   /// The [message], [textDirection], [location], and [layoutDirection]
   /// arguments must not be null.
   BannerPainter({
-    @required this.message,
-    @required this.textDirection,
-    @required this.location,
-    @required this.layoutDirection,
+    required this.message,
+    required this.textDirection,
+    required this.location,
+    required this.layoutDirection,
     this.color = _kColor,
     this.textStyle = _kTextStyle,
   }) : assert(message != null),
@@ -72,7 +70,7 @@ class BannerPainter extends CustomPainter {
        assert(location != null),
        assert(color != null),
        assert(textStyle != null),
-       super(repaint: PaintingBinding.instance.systemFonts);
+       super(repaint: PaintingBinding.instance!.systemFonts);
 
   /// The message to show in the banner.
   final String message;
@@ -120,9 +118,9 @@ class BannerPainter extends CustomPainter {
   );
 
   bool _prepared = false;
-  TextPainter _textPainter;
-  Paint _paintShadow;
-  Paint _paintBanner;
+  late TextPainter _textPainter;
+  late Paint _paintShadow;
+  late Paint _paintBanner;
 
   void _prepare() {
     _paintShadow = _shadow.toPaint();
@@ -176,7 +174,6 @@ class BannerPainter extends CustomPainter {
           case BannerLocation.topStart:
             return width;
         }
-        break;
       case TextDirection.ltr:
         switch (location) {
           case BannerLocation.bottomEnd:
@@ -188,9 +185,7 @@ class BannerPainter extends CustomPainter {
           case BannerLocation.topStart:
             return 0.0;
         }
-        break;
     }
-    return null;
   }
 
   double _translationY(double height) {
@@ -203,7 +198,6 @@ class BannerPainter extends CustomPainter {
       case BannerLocation.topEnd:
         return 0.0;
     }
-    return null;
   }
 
   double get _rotation {
@@ -219,7 +213,6 @@ class BannerPainter extends CustomPainter {
           case BannerLocation.topStart:
             return math.pi / 4.0;
         }
-        break;
       case TextDirection.ltr:
         switch (location) {
           case BannerLocation.bottomStart:
@@ -229,9 +222,7 @@ class BannerPainter extends CustomPainter {
           case BannerLocation.topStart:
             return -math.pi / 4.0;
         }
-        break;
     }
-    return null;
   }
 }
 
@@ -249,11 +240,11 @@ class Banner extends StatelessWidget {
   ///
   /// The [message] and [location] arguments must not be null.
   const Banner({
-    Key key,
+    Key? key,
     this.child,
-    @required this.message,
+    required this.message,
     this.textDirection,
-    @required this.location,
+    required this.location,
     this.layoutDirection,
     this.color = _kColor,
     this.textStyle = _kTextStyle,
@@ -266,7 +257,7 @@ class Banner extends StatelessWidget {
   /// The widget to show behind the banner.
   ///
   /// {@macro flutter.widgets.child}
-  final Widget child;
+  final Widget? child;
 
   /// The message to show in the banner.
   final String message;
@@ -285,7 +276,7 @@ class Banner extends StatelessWidget {
   /// See also:
   ///
   ///  * [layoutDirection], which controls the interpretation of the [location].
-  final TextDirection textDirection;
+  final TextDirection? textDirection;
 
   /// Where to show the banner (e.g., the upper right corner).
   final BannerLocation location;
@@ -299,7 +290,7 @@ class Banner extends StatelessWidget {
   /// See also:
   ///
   ///  * [textDirection], which controls the reading direction of the [message].
-  final TextDirection layoutDirection;
+  final TextDirection? layoutDirection;
 
   /// The color of the banner.
   final Color color;
@@ -313,9 +304,9 @@ class Banner extends StatelessWidget {
     return CustomPaint(
       foregroundPainter: BannerPainter(
         message: message,
-        textDirection: textDirection ?? Directionality.of(context),
+        textDirection: textDirection ?? Directionality.of(context)!,
         location: location,
-        layoutDirection: layoutDirection ?? Directionality.of(context),
+        layoutDirection: layoutDirection ?? Directionality.of(context)!,
         color: color,
         textStyle: textStyle,
       ),
@@ -331,7 +322,7 @@ class Banner extends StatelessWidget {
     properties.add(EnumProperty<BannerLocation>('location', location));
     properties.add(EnumProperty<TextDirection>('layoutDirection', layoutDirection, defaultValue: null));
     properties.add(ColorProperty('color', color, showName: false));
-    textStyle?.debugFillProperties(properties, prefix: 'text ');
+    textStyle.debugFillProperties(properties, prefix: 'text ');
   }
 }
 
@@ -341,8 +332,8 @@ class Banner extends StatelessWidget {
 class CheckedModeBanner extends StatelessWidget {
   /// Creates a const checked mode banner.
   const CheckedModeBanner({
-    Key key,
-    @required this.child,
+    Key? key,
+    required this.child,
   }) : super(key: key);
 
   /// The widget to show behind the banner.

packages/flutter/lib/src/widgets/bottom_navigation_bar_item.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 Color;
 
 import 'framework.dart';
@@ -25,14 +23,14 @@ class BottomNavigationBarItem {
   ///
   /// The argument [icon] should not be null and the argument [title] should not be null when used in a Material Design's [BottomNavigationBar].
   const BottomNavigationBarItem({
-    @required this.icon,
+    required this.icon,
     @Deprecated(
       'Use "label" instead, as it allows for an improved text-scaling experience. '
       'This feature was deprecated after v1.19.0.'
     )
     this.title,
     this.label,
-    Widget activeIcon,
+    Widget? activeIcon,
     this.backgroundColor,
   }) : activeIcon = activeIcon ?? icon,
        assert(label == null || title == null),
@@ -75,14 +73,14 @@ class BottomNavigationBarItem {
     'Use "label" instead, as it allows for an improved text-scaling experience. '
     'This feature was deprecated after v1.19.0.'
   )
-  final Widget title;
+  final Widget? title;
 
   /// The text label for this [BottomNavigationBarItem].
   ///
   /// This will be used to create a [Text] widget to put in the bottom navigation bar,
   /// and in Material Design [BottomNavigationBar]s, this will be used to display
   /// a tooltip on long press of an item in the [BottomNavigationBar].
-  final String label;
+  final String? label;
 
   /// The color of the background radial animation for material [BottomNavigationBar].
   ///
@@ -97,5 +95,5 @@ class BottomNavigationBarItem {
   ///
   ///  * [Icon.color] and [ImageIcon.color] to control the foreground color of
   ///    the icons themselves.
-  final Color backgroundColor;
+  final Color? backgroundColor;
 }

packages/flutter/lib/src/widgets/color_filter.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';
 
 import 'package:flutter/foundation.dart';
@@ -19,7 +17,7 @@ class ColorFiltered extends SingleChildRenderObjectWidget {
   /// Creates a widget that applies a [ColorFilter] to its child.
   ///
   /// The [colorFilter] must not be null.
-  const ColorFiltered({@required this.colorFilter, Widget child, Key key})
+  const ColorFiltered({required this.colorFilter, Widget? child, Key? key})
       : assert(colorFilter != null),
         super(key: key, child: child);
 

packages/flutter/lib/src/widgets/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
-
 /// The minimum dimension of any interactive region according to the Material
 /// guidelines.
 ///

packages/flutter/lib/src/widgets/container.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:flutter/foundation.dart';
 import 'package:flutter/painting.dart';
 import 'package:flutter/rendering.dart';
@@ -61,10 +59,10 @@ class DecoratedBox extends SingleChildRenderObjectWidget {
   /// The [decoration] and [position] arguments must not be null. By default the
   /// decoration paints behind the child.
   const DecoratedBox({
-    Key key,
-    @required this.decoration,
+    Key? key,
+    required this.decoration,
     this.position = DecorationPosition.background,
-    Widget child,
+    Widget? child,
   }) : assert(decoration != null),
        assert(position != null),
        super(key: key, child: child);
@@ -98,7 +96,6 @@ class DecoratedBox extends SingleChildRenderObjectWidget {
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     String label;
-    if (position != null) {
     switch (position) {
       case DecorationPosition.background:
         label = 'bg';
@@ -107,10 +104,7 @@ class DecoratedBox extends SingleChildRenderObjectWidget {
         label = 'fg';
         break;
     }
-    } else {
-      label = 'decoration';
-    }
-    properties.add(EnumProperty<DecorationPosition>('position', position, level: position != null ? DiagnosticLevel.hidden : DiagnosticLevel.info));
+    properties.add(EnumProperty<DecorationPosition>('position', position, level: DiagnosticLevel.hidden));
     properties.add(DiagnosticsProperty<Decoration>(
       label,
       decoration,
@@ -263,15 +257,15 @@ class Container extends StatelessWidget {
   /// color. To supply a decoration with a color, use `decoration:
   /// BoxDecoration(color: color)`.
   Container({
-    Key key,
+    Key? key,
     this.alignment,
     this.padding,
     this.color,
     this.decoration,
     this.foregroundDecoration,
-    double width,
-    double height,
-    BoxConstraints constraints,
+    double? width,
+    double? height,
+    BoxConstraints? constraints,
     this.margin,
     this.transform,
     this.child,
@@ -301,7 +295,7 @@ class Container extends StatelessWidget {
   /// will attempt to be as small as possible.
   ///
   /// {@macro flutter.widgets.child}
-  final Widget child;
+  final Widget? child;
 
   /// Align the [child] within the container.
   ///
@@ -317,14 +311,14 @@ class Container extends StatelessWidget {
   ///    specify an [AlignmentGeometry].
   ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
   ///    relative to text direction.
-  final AlignmentGeometry alignment;
+  final AlignmentGeometry? alignment;
 
   /// Empty space to inscribe inside the [decoration]. The [child], if any, is
   /// placed inside this padding.
   ///
   /// This padding is in addition to any padding inherent in the [decoration];
   /// see [Decoration.padding].
-  final EdgeInsetsGeometry padding;
+  final EdgeInsetsGeometry? padding;
 
   /// The color to paint behind the [child].
   ///
@@ -335,7 +329,7 @@ class Container extends StatelessWidget {
   /// If the [decoration] is used, this property must be null. A background
   /// color may still be painted by the [decoration] even if this property is
   /// null.
-  final Color color;
+  final Color? color;
 
   /// The decoration to paint behind the [child].
   ///
@@ -343,10 +337,10 @@ class Container extends StatelessWidget {
   ///
   /// The [child] is not clipped to the decoration. To clip a child to the shape
   /// of a particular [ShapeDecoration], consider using a [ClipPath] widget.
-  final Decoration decoration;
+  final Decoration? decoration;
 
   /// The decoration to paint in front of the [child].
-  final Decoration foregroundDecoration;
+  final Decoration? foregroundDecoration;
 
   /// Additional constraints to apply to the child.
   ///
@@ -354,13 +348,13 @@ class Container extends StatelessWidget {
   /// `constraints` argument to set this property.
   ///
   /// The [padding] goes inside the constraints.
-  final BoxConstraints constraints;
+  final BoxConstraints? constraints;
 
   /// Empty space to surround the [decoration] and [child].
-  final EdgeInsetsGeometry margin;
+  final EdgeInsetsGeometry? margin;
 
   /// The transformation matrix to apply before painting the container.
-  final Matrix4 transform;
+  final Matrix4? transform;
 
   /// The clip behavior when [Container.decoration] is not null.
   ///
@@ -372,20 +366,20 @@ class Container extends StatelessWidget {
   /// method throws an [UnsupportedError].)
   final Clip clipBehavior;
 
-  EdgeInsetsGeometry get _paddingIncludingDecoration {
-    if (decoration == null || decoration.padding == null)
+  EdgeInsetsGeometry? get _paddingIncludingDecoration {
+    if (decoration == null || decoration!.padding == null)
       return padding;
-    final EdgeInsetsGeometry decorationPadding = decoration.padding;
+    final EdgeInsetsGeometry? decorationPadding = decoration!.padding;
     if (padding == null)
       return decorationPadding;
-    return padding.add(decorationPadding);
+    return padding!.add(decorationPadding!);
   }
 
   @override
   Widget build(BuildContext context) {
-    Widget current = child;
+    Widget? current = child;
 
-    if (child == null && (constraints == null || !constraints.isTight)) {
+    if (child == null && (constraints == null || !constraints!.isTight)) {
       current = LimitedBox(
         maxWidth: 0.0,
         maxHeight: 0.0,
@@ -394,21 +388,21 @@ class Container extends StatelessWidget {
     }
 
     if (alignment != null)
-      current = Align(alignment: alignment, child: current);
+      current = Align(alignment: alignment!, child: current);
 
-    final EdgeInsetsGeometry effectivePadding = _paddingIncludingDecoration;
+    final EdgeInsetsGeometry? effectivePadding = _paddingIncludingDecoration;
     if (effectivePadding != null)
       current = Padding(padding: effectivePadding, child: current);
 
     if (color != null)
-      current = ColoredBox(color: color, child: current);
+      current = ColoredBox(color: color!, child: current);
 
     if (clipBehavior != Clip.none) {
       assert(decoration != null);
       current = ClipPath(
         clipper: _DecorationClipper(
           textDirection: Directionality.of(context),
-          decoration: decoration
+          decoration: decoration!,
         ),
         clipBehavior: clipBehavior,
         child: current,
@@ -416,26 +410,26 @@ class Container extends StatelessWidget {
     }
 
     if (decoration != null)
-      current = DecoratedBox(decoration: decoration, child: current);
+      current = DecoratedBox(decoration: decoration!, child: current);
 
     if (foregroundDecoration != null) {
       current = DecoratedBox(
-        decoration: foregroundDecoration,
+        decoration: foregroundDecoration!,
         position: DecorationPosition.foreground,
         child: current,
       );
     }
 
     if (constraints != null)
-      current = ConstrainedBox(constraints: constraints, child: current);
+      current = ConstrainedBox(constraints: constraints!, child: current);
 
     if (margin != null)
-      current = Padding(padding: margin, child: current);
+      current = Padding(padding: margin!, child: current);
 
     if (transform != null)
-      current = Transform(transform: transform, child: current);
+      current = Transform(transform: transform!, child: current);
 
-    return current;
+    return current!;
   }
 
   @override
@@ -458,8 +452,8 @@ class Container extends StatelessWidget {
 /// A clipper that uses [Decoration.getClipPath] to clip.
 class _DecorationClipper extends CustomClipper<Path> {
   _DecorationClipper({
-    TextDirection textDirection,
-    @required this.decoration
+    TextDirection? textDirection,
+    required this.decoration
   }) : assert(decoration != null),
        textDirection = textDirection ?? TextDirection.ltr;
 

packages/flutter/lib/src/widgets/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:collection';
 import 'dart:developer' show Timeline; // to disambiguate reference in dartdocs below
 
@@ -55,7 +53,7 @@ typedef RebuildDirtyWidgetCallback = void Function(Element e, bool builtOnce);
 ///    rebuilds occurred when the
 ///    `ext.flutter.inspector.trackRebuildDirtyWidgets` service extension is
 ///    enabled.
-RebuildDirtyWidgetCallback debugOnRebuildDirtyWidget;
+RebuildDirtyWidgetCallback? debugOnRebuildDirtyWidget;
 
 /// Log all calls to [BuildOwner.buildScope].
 ///
@@ -110,13 +108,13 @@ bool debugProfileBuildsEnabled = false;
 /// Show banners for deprecated widgets.
 bool debugHighlightDeprecatedWidgets = false;
 
-Key _firstNonUniqueKey(Iterable<Widget> widgets) {
+Key? _firstNonUniqueKey(Iterable<Widget> widgets) {
   final Set<Key> keySet = HashSet<Key>();
   for (final Widget widget in widgets) {
     assert(widget != null);
     if (widget.key == null)
       continue;
-    if (!keySet.add(widget.key))
+    if (!keySet.add(widget.key!))
       return widget.key;
   }
   return null;
@@ -138,7 +136,7 @@ Key _firstNonUniqueKey(Iterable<Widget> widgets) {
 /// Does nothing if asserts are disabled. Always returns true.
 bool debugChildrenHaveDuplicateKeys(Widget parent, Iterable<Widget> children) {
   assert(() {
-    final Key nonUniqueKey = _firstNonUniqueKey(children);
+    final Key? nonUniqueKey = _firstNonUniqueKey(children);
     if (nonUniqueKey != null) {
       throw FlutterError(
         'Duplicate keys found.\n'
@@ -165,7 +163,7 @@ bool debugChildrenHaveDuplicateKeys(Widget parent, Iterable<Widget> children) {
 /// Does nothing if asserts are disabled. Always returns true.
 bool debugItemsHaveDuplicateKeys(Iterable<Widget> items) {
   assert(() {
-    final Key nonUniqueKey = _firstNonUniqueKey(items);
+    final Key? nonUniqueKey = _firstNonUniqueKey(items);
     if (nonUniqueKey != null)
       throw FlutterError('Duplicate key found: $nonUniqueKey.');
     return true;
@@ -262,7 +260,7 @@ bool debugCheckHasMediaQuery(BuildContext context) {
 /// with the more generic advice regarding [Directionality].
 ///
 /// Does nothing if asserts are disabled. Always returns true.
-bool debugCheckHasDirectionality(BuildContext context, { String why, String hint, String alternative }) {
+bool debugCheckHasDirectionality(BuildContext context, { String? why, String? hint, String? alternative }) {
   assert(() {
     if (context.widget is! Directionality && context.findAncestorWidgetOfExactType<Directionality>() == null) {
       why = why == null ? '' : ' $why';
@@ -296,7 +294,7 @@ bool debugCheckHasDirectionality(BuildContext context, { String why, String hint
 /// function returned a non-null value, as typically required.
 ///
 /// Does nothing when asserts are disabled.
-void debugWidgetBuilderValue(Widget widget, Widget built) {
+void debugWidgetBuilderValue(Widget widget, Widget? built) {
   assert(() {
     if (built == null) {
       throw FlutterError.fromParts(<DiagnosticsNode>[

packages/flutter/lib/src/widgets/disposable_build_context.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'framework.dart';
 
 /// Provides non-leaking access to a [BuildContext].
@@ -31,21 +29,21 @@ class DisposableBuildContext<T extends State> {
   /// The [State] must not be null, and [State.mounted] must be true.
   DisposableBuildContext(this._state)
       : assert(_state != null),
-        assert(_state.mounted, 'A DisposableBuildContext was given a BuildContext for an Element that is not mounted.');
+        assert(_state!.mounted, 'A DisposableBuildContext was given a BuildContext for an Element that is not mounted.');
 
-  T _state;
+  T? _state;
 
   /// Provides safe access to the build context.
   ///
   /// If [dispose] has been called, will return null.
   ///
   /// Otherwise, asserts the [_state] is still mounted and returns its context.
-  BuildContext get context {
+  BuildContext? get context {
     assert(_debugValidate());
     if (_state == null) {
       return null;
     }
-    return _state.context;
+    return _state!.context;
   }
 
   /// Called from asserts or tests to determine whether this object is in a
@@ -55,7 +53,7 @@ class DisposableBuildContext<T extends State> {
   /// but the state this is tracking is unmounted.
   bool _debugValidate() {
     assert(
-      _state == null || _state.mounted,
+      _state == null || _state!.mounted,
       'A DisposableBuildContext tried to access the BuildContext of a disposed '
       'State object. This can happen when the creator of this '
       'DisposableBuildContext fails to call dispose when it is disposed.',

packages/flutter/lib/src/widgets/dual_transition_builder.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 'basic.dart';
 import 'framework.dart';
 
@@ -15,8 +13,8 @@ import 'framework.dart';
 /// The `animation` provided to the builder always runs forward from 0.0 to 1.0.
 typedef AnimatedTransitionBuilder = Widget Function(
   BuildContext context,
-  Animation<double> animation,
-  Widget child,
+  Animation<double?> animation,
+  Widget? child,
 );
 
 /// A transition builder that animates its [child] based on the
@@ -38,10 +36,10 @@ class DualTransitionBuilder extends StatefulWidget {
   /// The [animation], [forwardBuilder], and [reverseBuilder] arguments are
   /// required and must not be null.
   const DualTransitionBuilder({
-    Key key,
-    @required this.animation,
-    @required this.forwardBuilder,
-    @required this.reverseBuilder,
+    Key? key,
+    required this.animation,
+    required this.forwardBuilder,
+    required this.reverseBuilder,
     this.child,
   }) : assert(animation != null),
        assert(forwardBuilder != null),
@@ -89,14 +87,14 @@ class DualTransitionBuilder extends StatefulWidget {
   ///
   /// This child widget will be wrapped by the transitions built by
   /// [forwardBuilder] and [reverseBuilder].
-  final Widget child;
+  final Widget? child;
 
   @override
   State<DualTransitionBuilder> createState() => _DualTransitionBuilderState();
 }
 
 class _DualTransitionBuilderState extends State<DualTransitionBuilder> {
-  AnimationStatus _effectiveAnimationStatus;
+  late AnimationStatus _effectiveAnimationStatus;
   final ProxyAnimation _forwardAnimation = ProxyAnimation();
   final ProxyAnimation _reverseAnimation = ProxyAnimation();
 
@@ -134,8 +132,8 @@ class _DualTransitionBuilderState extends State<DualTransitionBuilder> {
   // yield a disjoint experience since the forward and reverse transitions are
   // very different.
   AnimationStatus _calculateEffectiveAnimationStatus({
-    @required AnimationStatus lastEffective,
-    @required AnimationStatus current,
+    required AnimationStatus lastEffective,
+    required AnimationStatus current,
   }) {
     assert(current != null);
     assert(lastEffective != null);
@@ -152,7 +150,6 @@ class _DualTransitionBuilderState extends State<DualTransitionBuilder> {
           case AnimationStatus.reverse:
             return lastEffective;
         }
-        break;
       case AnimationStatus.reverse:
         switch (lastEffective) {
           case AnimationStatus.dismissed:
@@ -162,9 +159,7 @@ class _DualTransitionBuilderState extends State<DualTransitionBuilder> {
           case AnimationStatus.forward:
             return lastEffective;
         }
-        break;
     }
-    return null; // unreachable
   }
 
   void _updateAnimations() {

packages/flutter/lib/src/widgets/grid_paper.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:flutter/rendering.dart';
 
 import 'basic.dart';
@@ -11,10 +9,10 @@ import 'framework.dart';
 
 class _GridPaperPainter extends CustomPainter {
   const _GridPaperPainter({
-    this.color,
-    this.interval,
-    this.divisions,
-    this.subdivisions,
+    required this.color,
+    required this.interval,
+    required this.divisions,
+    required this.subdivisions,
   });
 
   final Color color;
@@ -60,7 +58,7 @@ class _GridPaperPainter extends CustomPainter {
 class GridPaper extends StatelessWidget {
   /// Creates a widget that draws a rectilinear grid of 1-pixel-wide lines.
   const GridPaper({
-    Key key,
+    Key? key,
     this.color = const Color(0x7FC3E8F3),
     this.interval = 100.0,
     this.divisions = 2,
@@ -105,7 +103,7 @@ class GridPaper extends StatelessWidget {
   /// The widget below this widget in the tree.
   ///
   /// {@macro flutter.widgets.child}
-  final Widget child;
+  final Widget? child;
 
   @override
   Widget build(BuildContext context) {