migrate some cupertino files to nullsafety (#66024)

* migrate some cupertino files to nullsafety

* address review comments

* address review comments

packages/flutter/lib/src/cupertino/activity_indicator.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/widgets.dart';
@@ -28,7 +26,7 @@ const Color _kActiveTickColor = CupertinoDynamicColor.withBrightness(
 class CupertinoActivityIndicator extends StatefulWidget {
   /// Creates an iOS-style activity indicator that spins clockwise.
   const CupertinoActivityIndicator({
-    Key key,
+    Key? key,
     this.animating = true,
     this.radius = _kDefaultIndicatorRadius,
   })  : assert(animating != null),
@@ -44,7 +42,7 @@ class CupertinoActivityIndicator extends StatefulWidget {
   /// will be shown) and 1.0 (all ticks will be shown) inclusive. Defaults
   /// to 1.0.
   const CupertinoActivityIndicator.partiallyRevealed({
-    Key key,
+    Key? key,
     this.radius = _kDefaultIndicatorRadius,
     this.progress = 1.0,
   })  : assert(radius != null),
@@ -80,7 +78,7 @@ class CupertinoActivityIndicator extends StatefulWidget {
 
 class _CupertinoActivityIndicatorState extends State<CupertinoActivityIndicator>
     with SingleTickerProviderStateMixin {
-  AnimationController _controller;
+  late AnimationController _controller;
 
   @override
   void initState() {
@@ -121,7 +119,7 @@ class _CupertinoActivityIndicatorState extends State<CupertinoActivityIndicator>
         painter: _CupertinoActivityIndicatorPainter(
           position: _controller,
           activeColor:
-              CupertinoDynamicColor.resolve(_kActiveTickColor, context),
+              CupertinoDynamicColor.resolve(_kActiveTickColor, context)!,
           radius: widget.radius,
           progress: widget.progress,
         ),
@@ -150,10 +148,10 @@ const int _partiallyRevealedAlpha = 147;
 
 class _CupertinoActivityIndicatorPainter extends CustomPainter {
   _CupertinoActivityIndicatorPainter({
-    @required this.position,
-    @required this.activeColor,
-    @required this.radius,
-    @required this.progress,
+    required this.position,
+    required this.activeColor,
+    required this.radius,
+    required this.progress,
   })  : tickFundamentalRRect = RRect.fromLTRBXY(
           -radius / _kDefaultIndicatorRadius,
           -radius / 3.0,

packages/flutter/lib/src/cupertino/button.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/widgets.dart';
 
@@ -34,15 +32,15 @@ const EdgeInsets _kBackgroundButtonPadding = EdgeInsets.symmetric(
 class CupertinoButton extends StatefulWidget {
   /// Creates an iOS-style button.
   const CupertinoButton({
-    Key key,
-    @required this.child,
+    Key? key,
+    required this.child,
     this.padding,
     this.color,
     this.disabledColor = CupertinoColors.quaternarySystemFill,
     this.minSize = kMinInteractiveDimensionCupertino,
     this.pressedOpacity = 0.4,
     this.borderRadius = const BorderRadius.all(Radius.circular(8.0)),
-    @required this.onPressed,
+    required this.onPressed,
   }) : assert(pressedOpacity == null || (pressedOpacity >= 0.0 && pressedOpacity <= 1.0)),
        assert(disabledColor != null),
        _filled = false,
@@ -55,14 +53,14 @@ class CupertinoButton extends StatefulWidget {
   /// To specify a custom background color, use the [color] argument of the
   /// default constructor.
   const CupertinoButton.filled({
-    Key key,
-    @required this.child,
+    Key? key,
+    required this.child,
     this.padding,
     this.disabledColor = CupertinoColors.quaternarySystemFill,
     this.minSize = kMinInteractiveDimensionCupertino,
     this.pressedOpacity = 0.4,
     this.borderRadius = const BorderRadius.all(Radius.circular(8.0)),
-    @required this.onPressed,
+    required this.onPressed,
   }) : assert(pressedOpacity == null || (pressedOpacity >= 0.0 && pressedOpacity <= 1.0)),
        assert(disabledColor != null),
        color = null,
@@ -77,7 +75,7 @@ class CupertinoButton extends StatefulWidget {
   /// The amount of space to surround the child inside the bounds of the button.
   ///
   /// Defaults to 16.0 pixels.
-  final EdgeInsetsGeometry padding;
+  final EdgeInsetsGeometry? padding;
 
   /// The color of the button's background.
   ///
@@ -85,7 +83,7 @@ class CupertinoButton extends StatefulWidget {
   ///
   /// Defaults to the [CupertinoTheme]'s `primaryColor` when the
   /// [CupertinoButton.filled] constructor is used.
-  final Color color;
+  final Color? color;
 
   /// The color of the button's background when the button is disabled.
   ///
@@ -98,25 +96,25 @@ class CupertinoButton extends StatefulWidget {
   /// The callback that is called when the button is tapped or otherwise activated.
   ///
   /// If this is set to null, the button will be disabled.
-  final VoidCallback onPressed;
+  final VoidCallback? onPressed;
 
   /// Minimum size of the button.
   ///
   /// Defaults to kMinInteractiveDimensionCupertino which the iOS Human
   /// Interface Guidelines recommends as the minimum tappable area.
-  final double minSize;
+  final double? minSize;
 
   /// The opacity that the button will fade to when it is pressed.
   /// The button will have an opacity of 1.0 when it is not pressed.
   ///
   /// This defaults to 0.4. If null, opacity will not change on pressed if using
   /// your own custom effects is desired.
-  final double pressedOpacity;
+  final double? pressedOpacity;
 
   /// The radius of the button's corners when it has a background color.
   ///
   /// Defaults to round corners of 8 logical pixels.
-  final BorderRadius borderRadius;
+  final BorderRadius? borderRadius;
 
   final bool _filled;
 
@@ -140,8 +138,8 @@ class _CupertinoButtonState extends State<CupertinoButton> with SingleTickerProv
   static const Duration kFadeInDuration = Duration(milliseconds: 100);
   final Tween<double> _opacityTween = Tween<double>(begin: 1.0);
 
-  AnimationController _animationController;
-  Animation<double> _opacityAnimation;
+  late AnimationController _animationController;
+  late Animation<double> _opacityAnimation;
 
   @override
   void initState() {
@@ -170,7 +168,6 @@ class _CupertinoButtonState extends State<CupertinoButton> with SingleTickerProv
   @override
   void dispose() {
     _animationController.dispose();
-    _animationController = null;
     super.dispose();
   }
 
@@ -214,12 +211,12 @@ class _CupertinoButtonState extends State<CupertinoButton> with SingleTickerProv
   Widget build(BuildContext context) {
     final bool enabled = widget.enabled;
     final CupertinoThemeData themeData = CupertinoTheme.of(context);
-    final Color primaryColor = themeData.primaryColor;
-    final Color backgroundColor = widget.color == null
+    final Color? primaryColor = themeData.primaryColor;
+    final Color? backgroundColor = widget.color == null
       ? (widget._filled ? primaryColor : null)
       : CupertinoDynamicColor.resolve(widget.color, context);
 
-    final Color foregroundColor = backgroundColor != null
+    final Color? foregroundColor = backgroundColor != null
       ? themeData.primaryContrastingColor
       : enabled
         ? primaryColor
@@ -239,9 +236,9 @@ class _CupertinoButtonState extends State<CupertinoButton> with SingleTickerProv
           constraints: widget.minSize == null
             ? const BoxConstraints()
             : BoxConstraints(
-              minWidth: widget.minSize,
-              minHeight: widget.minSize,
-            ),
+                minWidth: widget.minSize!,
+                minHeight: widget.minSize!,
+              ),
           child: FadeTransition(
             opacity: _opacityAnimation,
             child: DecoratedBox(

packages/flutter/lib/src/cupertino/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 iOS Human
 /// Interface Guidelines.
 ///

packages/flutter/lib/src/cupertino/icons.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/widgets.dart';
 
 /// Identifiers for the supported Cupertino icons.

packages/flutter/lib/src/cupertino/picker.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';
 import 'package:flutter/services.dart';
@@ -70,7 +68,7 @@ class CupertinoPicker extends StatefulWidget {
   /// will loop the list back to the beginning.  If set to false, the list will
   /// stop scrolling when you reach the end or the beginning.
   CupertinoPicker({
-    Key key,
+    Key? key,
     this.diameterRatio = _kDefaultDiameterRatio,
     this.backgroundColor,
     this.offAxisFraction = 0.0,
@@ -78,9 +76,9 @@ class CupertinoPicker extends StatefulWidget {
     this.magnification = 1.0,
     this.scrollController,
     this.squeeze = _kSqueeze,
-    @required this.itemExtent,
-    @required this.onSelectedItemChanged,
-    @required List<Widget> children,
+    required this.itemExtent,
+    required this.onSelectedItemChanged,
+    required List<Widget> children,
     bool looping = false,
   }) : assert(children != null),
        assert(diameterRatio != null),
@@ -113,7 +111,7 @@ class CupertinoPicker extends StatefulWidget {
   /// (i.e. the picker is going to have a completely transparent background), to match
   /// the native UIPicker and UIDatePicker.
   CupertinoPicker.builder({
-    Key key,
+    Key? key,
     this.diameterRatio = _kDefaultDiameterRatio,
     this.backgroundColor,
     this.offAxisFraction = 0.0,
@@ -121,10 +119,10 @@ class CupertinoPicker extends StatefulWidget {
     this.magnification = 1.0,
     this.scrollController,
     this.squeeze = _kSqueeze,
-    @required this.itemExtent,
-    @required this.onSelectedItemChanged,
-    @required IndexedWidgetBuilder itemBuilder,
-    int childCount,
+    required this.itemExtent,
+    required this.onSelectedItemChanged,
+    required NullableIndexedWidgetBuilder itemBuilder,
+    int? childCount,
   }) : assert(itemBuilder != null),
        assert(diameterRatio != null),
        assert(diameterRatio > 0.0, RenderListWheelViewport.diameterRatioZeroMessage),
@@ -153,7 +151,7 @@ class CupertinoPicker extends StatefulWidget {
   ///
   /// Any alpha value less 255 (fully opaque) will cause the removal of the
   /// wheel list edge fade gradient from rendering of the widget.
-  final Color backgroundColor;
+  final Color? backgroundColor;
 
   /// {@macro flutter.rendering.wheelList.offAxisFraction}
   final double offAxisFraction;
@@ -168,7 +166,7 @@ class CupertinoPicker extends StatefulWidget {
   /// to set the initial item.
   ///
   /// If null, an implicit one will be created internally.
-  final FixedExtentScrollController scrollController;
+  final FixedExtentScrollController? scrollController;
 
   /// The uniform height of all children.
   ///
@@ -198,8 +196,8 @@ class CupertinoPicker extends StatefulWidget {
 }
 
 class _CupertinoPickerState extends State<CupertinoPicker> {
-  int _lastHapticIndex;
-  FixedExtentScrollController _controller;
+  int? _lastHapticIndex;
+  FixedExtentScrollController? _controller;
 
   @override
   void initState() {
@@ -255,7 +253,7 @@ class _CupertinoPickerState extends State<CupertinoPicker> {
 
   /// Draws the magnifier borders.
   Widget _buildMagnifierScreen() {
-    final Color resolvedBorderColor = CupertinoDynamicColor.resolve(_kHighlighterBorder, context);
+    final Color resolvedBorderColor = CupertinoDynamicColor.resolve(_kHighlighterBorder, context)!;
 
     return IgnorePointer(
       child: Center(
@@ -276,7 +274,7 @@ class _CupertinoPickerState extends State<CupertinoPicker> {
 
   @override
   Widget build(BuildContext context) {
-    final Color resolvedBackgroundColor = CupertinoDynamicColor.resolve(widget.backgroundColor, context);
+    final Color? resolvedBackgroundColor = CupertinoDynamicColor.resolve(widget.backgroundColor, context);
 
     final Widget result = DefaultTextStyle(
       style: CupertinoTheme.of(context).textTheme.pickerTextStyle,
@@ -284,7 +282,7 @@ class _CupertinoPickerState extends State<CupertinoPicker> {
         children: <Widget>[
           Positioned.fill(
             child: _CupertinoPickerSemantics(
-              scrollController: widget.scrollController ?? _controller,
+              scrollController: widget.scrollController ?? _controller!,
               child: ListWheelScrollView.useDelegate(
                 controller: widget.scrollController ?? _controller,
                 physics: const FixedExtentScrollPhysics(),
@@ -321,38 +319,45 @@ class _CupertinoPickerState extends State<CupertinoPicker> {
 // scroll controller.
 class _CupertinoPickerSemantics extends SingleChildRenderObjectWidget {
   const _CupertinoPickerSemantics({
-    Key key,
-    Widget child,
-    @required this.scrollController,
+    Key? key,
+    Widget? child,
+    required this.scrollController,
   }) : super(key: key, child: child);
 
   final FixedExtentScrollController scrollController;
 
   @override
-  RenderObject createRenderObject(BuildContext context) => _RenderCupertinoPickerSemantics(scrollController, Directionality.of(context));
+  RenderObject createRenderObject(BuildContext context) {
+    assert(debugCheckHasDirectionality(context));
+    return _RenderCupertinoPickerSemantics(scrollController, Directionality.of(context)!);
+  }
 
   @override
   void updateRenderObject(BuildContext context, covariant _RenderCupertinoPickerSemantics renderObject) {
+    assert(debugCheckHasDirectionality(context));
     renderObject
-      ..textDirection = Directionality.of(context)
+      ..textDirection = Directionality.of(context)!
       ..controller = scrollController;
   }
 }
 
 class _RenderCupertinoPickerSemantics extends RenderProxyBox {
   _RenderCupertinoPickerSemantics(FixedExtentScrollController controller, this._textDirection) {
-    this.controller = controller;
+    _updateController(null, controller);
   }
 
   FixedExtentScrollController get controller => _controller;
-  FixedExtentScrollController _controller;
-  set controller(FixedExtentScrollController value) {
-    if (value == _controller)
+  late FixedExtentScrollController _controller;
+  set controller(FixedExtentScrollController value) => _updateController(_controller, value);
+
+  // This method exists to allow controller to be non-null. It is only called with a null oldValue from construtor.
+  void _updateController(FixedExtentScrollController? oldValue, FixedExtentScrollController value) {
+    if (value == oldValue)
       return;
-    if (_controller != null)
-      _controller.removeListener(_handleScrollUpdate);
+    if (oldValue != null)
+      oldValue.removeListener(_handleScrollUpdate);
     else
-      _currentIndex = value.initialItem ?? 0;
+      _currentIndex = value.initialItem;
     value.addListener(_handleScrollUpdate);
     _controller = value;
   }
@@ -399,15 +404,15 @@ class _RenderCupertinoPickerSemantics extends RenderProxyBox {
     final Map<int, SemanticsNode> indexedChildren = <int, SemanticsNode>{};
     scrollable.visitChildren((SemanticsNode child) {
       assert(child.indexInParent != null);
-      indexedChildren[child.indexInParent] = child;
+      indexedChildren[child.indexInParent!] = child;
       return true;
     });
     if (indexedChildren[_currentIndex] == null) {
       return node.updateWith(config: config);
     }
-    config.value = indexedChildren[_currentIndex].label;
-    final SemanticsNode previousChild = indexedChildren[_currentIndex - 1];
-    final SemanticsNode nextChild = indexedChildren[_currentIndex + 1];
+    config.value = indexedChildren[_currentIndex]!.label;
+    final SemanticsNode? previousChild = indexedChildren[_currentIndex - 1];
+    final SemanticsNode? nextChild = indexedChildren[_currentIndex + 1];
     if (nextChild != null) {
       config.increasedValue = nextChild.label;
       config.onIncrease = _handleIncrease;

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

@@ -2,8 +2,7 @@
 // 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:math';
 
 import 'package:flutter/rendering.dart';
@@ -18,10 +17,10 @@ const double _kActivityIndicatorMargin = 16.0;
 
 class _CupertinoSliverRefresh extends SingleChildRenderObjectWidget {
   const _CupertinoSliverRefresh({
-    Key key,
+    Key? key,
     this.refreshIndicatorLayoutExtent = 0.0,
     this.hasLayoutExtent = false,
-    Widget child,
+    Widget? child,
   }) : assert(refreshIndicatorLayoutExtent != null),
        assert(refreshIndicatorLayoutExtent >= 0.0),
        assert(hasLayoutExtent != null),
@@ -61,9 +60,9 @@ class _CupertinoSliverRefresh extends SingleChildRenderObjectWidget {
 class _RenderCupertinoSliverRefresh extends RenderSliver
     with RenderObjectWithChildMixin<RenderBox> {
   _RenderCupertinoSliverRefresh({
-    @required double refreshIndicatorExtent,
-    @required bool hasLayoutExtent,
-    RenderBox child,
+    required double refreshIndicatorExtent,
+    required bool hasLayoutExtent,
+    RenderBox? child,
   }) : assert(refreshIndicatorExtent != null),
        assert(refreshIndicatorExtent >= 0.0),
        assert(hasLayoutExtent != null),
@@ -135,7 +134,7 @@ class _RenderCupertinoSliverRefresh extends RenderSliver
     // Layout the child giving it the space of the currently dragged overscroll
     // which may or may not include a sliver layout extent space that it will
     // keep after the user lets go during the refresh process.
-    child.layout(
+    child!.layout(
       constraints.asBoxConstraints(
         maxExtent: layoutExtent
             // Plus only the overscrolled portion immediately preceding this
@@ -153,11 +152,11 @@ class _RenderCupertinoSliverRefresh extends RenderSliver
           // layoutExtent may be zero. Check layoutExtent also since even
           // with a layoutExtent, the indicator builder may decide to not
           // build anything.
-          max(child.size.height, layoutExtent) - constraints.scrollOffset,
+          max(child!.size.height, layoutExtent) - constraints.scrollOffset,
           0.0,
         ),
         maxPaintExtent: max(
-          max(child.size.height, layoutExtent) - constraints.scrollOffset,
+          max(child!.size.height, layoutExtent) - constraints.scrollOffset,
           0.0,
         ),
         layoutExtent: max(layoutExtent - constraints.scrollOffset, 0.0),
@@ -171,8 +170,8 @@ class _RenderCupertinoSliverRefresh extends RenderSliver
   @override
   void paint(PaintingContext paintContext, Offset offset) {
     if (constraints.overlap < 0.0 ||
-        constraints.scrollOffset + child.size.height > 0) {
-      paintContext.paintChild(child, offset);
+        constraints.scrollOffset + child!.size.height > 0) {
+      paintContext.paintChild(child!, offset);
     }
   }
 
@@ -345,7 +344,7 @@ class CupertinoSliverRefreshControl extends StatefulWidget {
   /// The [onRefresh] argument will be called when pulled far enough to trigger
   /// a refresh.
   const CupertinoSliverRefreshControl({
-    Key key,
+    Key? key,
     this.refreshTriggerPullDistance = _defaultRefreshTriggerPullDistance,
     this.refreshIndicatorExtent = _defaultRefreshIndicatorExtent,
     this.builder = buildRefreshIndicator,
@@ -389,7 +388,7 @@ class CupertinoSliverRefreshControl extends StatefulWidget {
   ///
   /// Will not be called when the available space is zero such as before any
   /// overscroll.
-  final RefreshControlIndicatorBuilder builder;
+  final RefreshControlIndicatorBuilder? builder;
 
   /// Callback invoked when pulled by [refreshTriggerPullDistance].
   ///
@@ -399,7 +398,7 @@ class CupertinoSliverRefreshControl extends StatefulWidget {
   /// Can be null, in which case a single frame of [RefreshIndicatorMode.armed]
   /// state will be drawn before going immediately to the [RefreshIndicatorMode.done]
   /// where the sliver will start retracting.
-  final RefreshCallback onRefresh;
+  final RefreshCallback? onRefresh;
 
   static const double _defaultRefreshTriggerPullDistance = 100.0;
   static const double _defaultRefreshIndicatorExtent = 60.0;
@@ -408,7 +407,7 @@ class CupertinoSliverRefreshControl extends StatefulWidget {
   /// state that gets passed into the [builder] function. Used for testing.
   @visibleForTesting
   static RefreshIndicatorMode state(BuildContext context) {
-    final _CupertinoSliverRefreshControlState state = context.findAncestorStateOfType<_CupertinoSliverRefreshControlState>();
+    final _CupertinoSliverRefreshControlState state = context.findAncestorStateOfType<_CupertinoSliverRefreshControlState>()!;
     return state.refreshState;
   }
 
@@ -483,9 +482,9 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
   // original `refreshTriggerPullDistance` is left.
   static const double _inactiveResetOverscrollFraction = 0.1;
 
-  RefreshIndicatorMode refreshState;
+  late RefreshIndicatorMode refreshState;
   // [Future] returned by the widget's `onRefresh`.
-  Future<void> refreshTask;
+  Future<void>? refreshTask;
   // The amount of space available from the inner indicator box's perspective.
   //
   // The value is the sum of the sliver's layout extent and the overscroll
@@ -512,10 +511,10 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
       nextState = RefreshIndicatorMode.done;
       // Either schedule the RenderSliver to re-layout on the next frame
       // when not currently in a frame or schedule it on the next frame.
-      if (SchedulerBinding.instance.schedulerPhase == SchedulerPhase.idle) {
+      if (SchedulerBinding.instance!.schedulerPhase == SchedulerPhase.idle) {
         setState(() => hasSliverLayoutExtent = false);
       } else {
-        SchedulerBinding.instance.addPostFrameCallback((Duration timestamp) {
+        SchedulerBinding.instance!.addPostFrameCallback((Duration timestamp) {
           setState(() => hasSliverLayoutExtent = false);
         });
       }
@@ -541,8 +540,8 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
             // Call onRefresh after this frame finished since the function is
             // user supplied and we're always here in the middle of the sliver's
             // performLayout.
-            SchedulerBinding.instance.addPostFrameCallback((Duration timestamp) {
-              refreshTask = widget.onRefresh()..whenComplete(() {
+            SchedulerBinding.instance!.addPostFrameCallback((Duration timestamp) {
+              refreshTask = widget.onRefresh!()..whenComplete(() {
                 if (mounted) {
                   setState(() => refreshTask = null);
                   // Trigger one more transition because by this time, BoxConstraint's
@@ -557,9 +556,6 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
           }
           return RefreshIndicatorMode.armed;
         }
-        // Don't continue here. We can never possibly call onRefresh and
-        // progress to the next state in one [computeNextState] call.
-        break;
       case RefreshIndicatorMode.armed:
         if (refreshState == RefreshIndicatorMode.armed && refreshTask == null) {
           goToDone();
@@ -610,7 +606,7 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
           latestIndicatorBoxExtent = constraints.maxHeight;
           refreshState = transitionNextState();
           if (widget.builder != null && latestIndicatorBoxExtent > 0) {
-            return widget.builder(
+            return widget.builder!(
               context,
               refreshState,
               latestIndicatorBoxExtent,

packages/flutter/lib/src/cupertino/thumb_painter.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/painting.dart';
 
 import 'colors.dart';

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

@@ -163,7 +163,7 @@ class ListWheelChildBuilderDelegate extends ListWheelChildDelegate {
   }) : assert(builder != null);
 
   /// Called lazily to build children.
-  final IndexedWidgetBuilder builder;
+  final NullableIndexedWidgetBuilder builder;
 
   /// {@template flutter.widgets.wheelList.childCount}
   /// If non-null, [childCount] is the maximum number of children that can be

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

@@ -633,7 +633,7 @@ class HeroControllerScope extends InheritedWidget {
   /// Creates a widget to host the input [controller].
   const HeroControllerScope({
     Key? key,
-    this.controller,
+    required HeroController this.controller,
     required Widget child,
   }) : assert(controller != null),
        super(key: key, child: child);