migrate some cupertino files to nullsafety (#66424)

* migrate some cupertino files to nullsafety

* address review comments

packages/flutter/lib/src/cupertino/action_sheet.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 ImageFilter;
 
 import 'package:flutter/foundation.dart';
@@ -113,7 +111,7 @@ class CupertinoActionSheet extends StatelessWidget {
   /// Generally, action sheets are used to give the user a choice between
   /// two or more choices for the current context.
   const CupertinoActionSheet({
-    Key key,
+    Key? key,
     this.title,
     this.message,
     this.actions,
@@ -129,37 +127,37 @@ class CupertinoActionSheet extends StatelessWidget {
   /// the font of the [title] is bold.
   ///
   /// Typically a [Text] widget.
-  final Widget title;
+  final Widget? title;
 
   /// An optional descriptive message that provides more details about the
   /// reason for the alert.
   ///
   /// Typically a [Text] widget.
-  final Widget message;
+  final Widget? message;
 
   /// The set of actions that are displayed for the user to select.
   ///
   /// Typically this is a list of [CupertinoActionSheetAction] widgets.
-  final List<Widget> actions;
+  final List<Widget>? actions;
 
   /// A scroll controller that can be used to control the scrolling of the
   /// [message] in the action sheet.
   ///
   /// This attribute is typically not needed, as alert messages should be
   /// short.
-  final ScrollController messageScrollController;
+  final ScrollController? messageScrollController;
 
   /// A scroll controller that can be used to control the scrolling of the
   /// [actions] in the action sheet.
   ///
   /// This attribute is typically not needed.
-  final ScrollController actionScrollController;
+  final ScrollController? actionScrollController;
 
   /// The optional cancel button that is grouped separately from the other
   /// actions.
   ///
   /// Typically this is an [CupertinoActionSheetAction] widget.
-  final Widget cancelButton;
+  final Widget? cancelButton;
 
   Widget _buildContent(BuildContext context) {
     final List<Widget> content = <Widget>[];
@@ -183,14 +181,14 @@ class CupertinoActionSheet extends StatelessWidget {
   }
 
   Widget _buildActions() {
-    if (actions == null || actions.isEmpty) {
+    if (actions == null || actions!.isEmpty) {
       return Container(
         height: 0.0,
       );
     }
     return Container(
       child: _CupertinoAlertActionSection(
-        children: actions,
+        children: actions!,
         scrollController: actionScrollController,
         hasCancelButton: cancelButton != null,
       ),
@@ -210,6 +208,8 @@ class CupertinoActionSheet extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    assert(debugCheckHasMediaQuery(context));
+
     final List<Widget> children = <Widget>[
       Flexible(child: ClipRRect(
           borderRadius: BorderRadius.circular(12.0),
@@ -225,12 +225,12 @@ class CupertinoActionSheet extends StatelessWidget {
       if (cancelButton != null) _buildCancelButton(),
     ];
 
-    final Orientation orientation = MediaQuery.of(context).orientation;
+    final Orientation orientation = MediaQuery.of(context)!.orientation;
     double actionSheetWidth;
     if (orientation == Orientation.portrait) {
-      actionSheetWidth = MediaQuery.of(context).size.width - (_kEdgeHorizontalPadding * 2);
+      actionSheetWidth = MediaQuery.of(context)!.size.width - (_kEdgeHorizontalPadding * 2);
     } else {
-      actionSheetWidth = MediaQuery.of(context).size.height - (_kEdgeHorizontalPadding * 2);
+      actionSheetWidth = MediaQuery.of(context)!.size.height - (_kEdgeHorizontalPadding * 2);
     }
 
     return SafeArea(
@@ -270,11 +270,11 @@ class CupertinoActionSheetAction extends StatelessWidget {
   ///
   /// The [child] and [onPressed] arguments must not be null.
   const CupertinoActionSheetAction({
-    Key key,
-    @required this.onPressed,
+    Key? key,
+    required this.onPressed,
     this.isDefaultAction = false,
     this.isDestructiveAction = false,
-    @required this.child,
+    required this.child,
   }) : assert(child != null),
        assert(onPressed != null),
        super(key: key);
@@ -340,11 +340,11 @@ class CupertinoActionSheetAction extends StatelessWidget {
 
 class _CupertinoActionSheetCancelButton extends StatefulWidget {
   const _CupertinoActionSheetCancelButton({
-    Key key,
+    Key? key,
     this.child,
   }) : super(key: key);
 
-  final Widget child;
+  final Widget? child;
 
   @override
   _CupertinoActionSheetCancelButtonState createState() => _CupertinoActionSheetCancelButtonState();
@@ -388,9 +388,9 @@ class _CupertinoActionSheetCancelButtonState extends State<_CupertinoActionSheet
 
 class _CupertinoAlertRenderWidget extends RenderObjectWidget {
   const _CupertinoAlertRenderWidget({
-    Key key,
-    @required this.contentSection,
-    @required this.actionsSection,
+    Key? key,
+    required this.contentSection,
+    required this.actionsSection,
   }) : super(key: key);
 
   final Widget contentSection;
@@ -398,16 +398,17 @@ class _CupertinoAlertRenderWidget extends RenderObjectWidget {
 
   @override
   RenderObject createRenderObject(BuildContext context) {
+    assert(debugCheckHasMediaQuery(context));
     return _RenderCupertinoAlert(
-      dividerThickness: _kDividerThickness / MediaQuery.of(context).devicePixelRatio,
-      dividerColor: CupertinoDynamicColor.resolve(_kButtonDividerColor, context),
+      dividerThickness: _kDividerThickness / MediaQuery.of(context)!.devicePixelRatio,
+      dividerColor: CupertinoDynamicColor.resolve(_kButtonDividerColor, context)!,
     );
   }
 
   @override
   void updateRenderObject(BuildContext context, _RenderCupertinoAlert renderObject) {
     super.updateRenderObject(context, renderObject);
-    renderObject.dividerColor = CupertinoDynamicColor.resolve(_kButtonDividerColor, context);
+    renderObject.dividerColor = CupertinoDynamicColor.resolve(_kButtonDividerColor, context)!;
   }
 
   @override
@@ -419,27 +420,27 @@ class _CupertinoAlertRenderWidget extends RenderObjectWidget {
 class _CupertinoAlertRenderElement extends RenderObjectElement {
   _CupertinoAlertRenderElement(_CupertinoAlertRenderWidget widget) : super(widget);
 
-  Element _contentElement;
-  Element _actionsElement;
+  Element? _contentElement;
+  Element? _actionsElement;
 
   @override
   _CupertinoAlertRenderWidget get widget => super.widget as _CupertinoAlertRenderWidget;
 
   @override
-  _RenderCupertinoAlert get renderObject => super.renderObject as _RenderCupertinoAlert;
+  _RenderCupertinoAlert? get renderObject => super.renderObject as _RenderCupertinoAlert?;
 
   @override
   void visitChildren(ElementVisitor visitor) {
     if (_contentElement != null) {
-      visitor(_contentElement);
+      visitor(_contentElement!);
     }
     if (_actionsElement != null) {
-      visitor(_actionsElement);
+      visitor(_actionsElement!);
     }
   }
 
   @override
-  void mount(Element parent, dynamic newSlot) {
+  void mount(Element? parent, dynamic newSlot) {
     super.mount(parent, newSlot);
     _contentElement = updateChild(_contentElement,
         widget.contentSection, _AlertSections.contentSection);
@@ -479,11 +480,11 @@ class _CupertinoAlertRenderElement extends RenderObjectElement {
 
   @override
   void removeRenderObjectChild(RenderObject child, _AlertSections slot) {
-    assert(child == renderObject.contentSection || child == renderObject.actionsSection);
-    if (renderObject.contentSection == child) {
-      renderObject.contentSection = null;
-    } else if (renderObject.actionsSection == child) {
-      renderObject.actionsSection = null;
+    assert(child == renderObject!.contentSection || child == renderObject!.actionsSection);
+    if (renderObject!.contentSection == child) {
+      renderObject!.contentSection = null;
+    } else if (renderObject!.actionsSection == child) {
+      renderObject!.actionsSection = null;
     }
   }
 
@@ -491,10 +492,10 @@ class _CupertinoAlertRenderElement extends RenderObjectElement {
     assert(slot != null);
     switch (slot) {
       case _AlertSections.contentSection:
-        renderObject.contentSection = child as RenderBox;
+        renderObject!.contentSection = child as RenderBox;
         break;
       case _AlertSections.actionsSection:
-        renderObject.actionsSection = child as RenderBox;
+        renderObject!.actionsSection = child as RenderBox;
         break;
     }
   }
@@ -520,10 +521,10 @@ class _CupertinoAlertRenderElement extends RenderObjectElement {
 // to take up any remaining space that was not consumed by the content section.
 class _RenderCupertinoAlert extends RenderBox {
   _RenderCupertinoAlert({
-    RenderBox contentSection,
-    RenderBox actionsSection,
+    RenderBox? contentSection,
+    RenderBox? actionsSection,
     double dividerThickness = 0.0,
-    @required Color dividerColor,
+    required Color dividerColor,
   }) : assert(dividerColor != null),
        _contentSection = contentSection,
        _actionsSection = actionsSection,
@@ -532,30 +533,30 @@ class _RenderCupertinoAlert extends RenderBox {
         ..color = dividerColor
         ..style = PaintingStyle.fill;
 
-  RenderBox get contentSection => _contentSection;
-  RenderBox _contentSection;
-  set contentSection(RenderBox newContentSection) {
+  RenderBox? get contentSection => _contentSection;
+  RenderBox? _contentSection;
+  set contentSection(RenderBox? newContentSection) {
     if (newContentSection != _contentSection) {
       if (null != _contentSection) {
-        dropChild(_contentSection);
+        dropChild(_contentSection!);
       }
       _contentSection = newContentSection;
       if (null != _contentSection) {
-        adoptChild(_contentSection);
+        adoptChild(_contentSection!);
       }
     }
   }
 
-  RenderBox get actionsSection => _actionsSection;
-  RenderBox _actionsSection;
-  set actionsSection(RenderBox newActionsSection) {
+  RenderBox? get actionsSection => _actionsSection;
+  RenderBox? _actionsSection;
+  set actionsSection(RenderBox? newActionsSection) {
     if (newActionsSection != _actionsSection) {
       if (null != _actionsSection) {
-        dropChild(_actionsSection);
+        dropChild(_actionsSection!);
       }
       _actionsSection = newActionsSection;
       if (null != _actionsSection) {
-        adoptChild(_actionsSection);
+        adoptChild(_actionsSection!);
       }
     }
   }
@@ -576,10 +577,10 @@ class _RenderCupertinoAlert extends RenderBox {
   void attach(PipelineOwner owner) {
     super.attach(owner);
     if (null != contentSection) {
-      contentSection.attach(owner);
+      contentSection!.attach(owner);
     }
     if (null != actionsSection) {
-      actionsSection.attach(owner);
+      actionsSection!.attach(owner);
     }
   }
 
@@ -587,20 +588,20 @@ class _RenderCupertinoAlert extends RenderBox {
   void detach() {
     super.detach();
     if (null != contentSection) {
-      contentSection.detach();
+      contentSection!.detach();
     }
     if (null != actionsSection) {
-      actionsSection.detach();
+      actionsSection!.detach();
     }
   }
 
   @override
   void redepthChildren() {
     if (null != contentSection) {
-      redepthChild(contentSection);
+      redepthChild(contentSection!);
     }
     if (null != actionsSection) {
-      redepthChild(actionsSection);
+      redepthChild(actionsSection!);
     }
   }
 
@@ -614,10 +615,10 @@ class _RenderCupertinoAlert extends RenderBox {
   @override
   void visitChildren(RenderObjectVisitor visitor) {
     if (contentSection != null) {
-      visitor(contentSection);
+      visitor(contentSection!);
     }
     if (actionsSection != null) {
-      visitor(actionsSection);
+      visitor(actionsSection!);
     }
   }
 
@@ -625,10 +626,10 @@ class _RenderCupertinoAlert extends RenderBox {
   List<DiagnosticsNode> debugDescribeChildren() {
     final List<DiagnosticsNode> value = <DiagnosticsNode>[];
     if (contentSection != null) {
-      value.add(contentSection.toDiagnosticsNode(name: 'content'));
+      value.add(contentSection!.toDiagnosticsNode(name: 'content'));
     }
     if (actionsSection != null) {
-      value.add(actionsSection.toDiagnosticsNode(name: 'actions'));
+      value.add(actionsSection!.toDiagnosticsNode(name: 'actions'));
     }
     return value;
   }
@@ -645,8 +646,8 @@ class _RenderCupertinoAlert extends RenderBox {
 
   @override
   double computeMinIntrinsicHeight(double width) {
-    final double contentHeight = contentSection.getMinIntrinsicHeight(width);
-    final double actionsHeight = actionsSection.getMinIntrinsicHeight(width);
+    final double contentHeight = contentSection!.getMinIntrinsicHeight(width);
+    final double actionsHeight = actionsSection!.getMinIntrinsicHeight(width);
     final bool hasDivider = contentHeight > 0.0 && actionsHeight > 0.0;
     double height = contentHeight + (hasDivider ? _dividerThickness : 0.0) + actionsHeight;
 
@@ -659,8 +660,8 @@ class _RenderCupertinoAlert extends RenderBox {
 
   @override
   double computeMaxIntrinsicHeight(double width) {
-    final double contentHeight = contentSection.getMaxIntrinsicHeight(width);
-    final double actionsHeight = actionsSection.getMaxIntrinsicHeight(width);
+    final double contentHeight = contentSection!.getMaxIntrinsicHeight(width);
+    final double actionsHeight = actionsSection!.getMaxIntrinsicHeight(width);
     final bool hasDivider = contentHeight > 0.0 && actionsHeight > 0.0;
     double height = contentHeight + (hasDivider ? _dividerThickness : 0.0) + actionsHeight;
 
@@ -674,25 +675,25 @@ class _RenderCupertinoAlert extends RenderBox {
   @override
   void performLayout() {
     final BoxConstraints constraints = this.constraints;
-    final bool hasDivider = contentSection.getMaxIntrinsicHeight(constraints.maxWidth) > 0.0
-        && actionsSection.getMaxIntrinsicHeight(constraints.maxWidth) > 0.0;
+    final bool hasDivider = contentSection!.getMaxIntrinsicHeight(constraints.maxWidth) > 0.0
+        && actionsSection!.getMaxIntrinsicHeight(constraints.maxWidth) > 0.0;
     final double dividerThickness = hasDivider ? _dividerThickness : 0.0;
 
-    final double minActionsHeight = actionsSection.getMinIntrinsicHeight(constraints.maxWidth);
+    final double minActionsHeight = actionsSection!.getMinIntrinsicHeight(constraints.maxWidth);
 
     // Size alert content.
-    contentSection.layout(
+    contentSection!.layout(
       constraints.deflate(EdgeInsets.only(bottom: minActionsHeight + dividerThickness)),
       parentUsesSize: true,
     );
-    final Size contentSize = contentSection.size;
+    final Size contentSize = contentSection!.size;
 
     // Size alert actions.
-    actionsSection.layout(
+    actionsSection!.layout(
       constraints.deflate(EdgeInsets.only(top: contentSize.height + dividerThickness)),
       parentUsesSize: true,
     );
-    final Size actionsSize = actionsSection.size;
+    final Size actionsSize = actionsSection!.size;
 
     // Calculate overall alert height.
     final double actionSheetHeight = contentSize.height + dividerThickness + actionsSize.height;
@@ -702,30 +703,30 @@ class _RenderCupertinoAlert extends RenderBox {
 
     // Set the position of the actions box to sit at the bottom of the alert.
     // The content box defaults to the top left, which is where we want it.
-    assert(actionsSection.parentData is MultiChildLayoutParentData);
-    final MultiChildLayoutParentData actionParentData = actionsSection.parentData as MultiChildLayoutParentData;
+    assert(actionsSection!.parentData is MultiChildLayoutParentData);
+    final MultiChildLayoutParentData actionParentData = actionsSection!.parentData as MultiChildLayoutParentData;
     actionParentData.offset = Offset(0.0, contentSize.height + dividerThickness);
   }
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    final MultiChildLayoutParentData contentParentData = contentSection.parentData as MultiChildLayoutParentData;
-    contentSection.paint(context, offset + contentParentData.offset);
+    final MultiChildLayoutParentData contentParentData = contentSection!.parentData as MultiChildLayoutParentData;
+    contentSection!.paint(context, offset + contentParentData.offset);
 
-    final bool hasDivider = contentSection.size.height > 0.0 && actionsSection.size.height > 0.0;
+    final bool hasDivider = contentSection!.size.height > 0.0 && actionsSection!.size.height > 0.0;
     if (hasDivider) {
       _paintDividerBetweenContentAndActions(context.canvas, offset);
     }
 
-    final MultiChildLayoutParentData actionsParentData = actionsSection.parentData as MultiChildLayoutParentData;
-    actionsSection.paint(context, offset + actionsParentData.offset);
+    final MultiChildLayoutParentData actionsParentData = actionsSection!.parentData as MultiChildLayoutParentData;
+    actionsSection!.paint(context, offset + actionsParentData.offset);
   }
 
   void _paintDividerBetweenContentAndActions(Canvas canvas, Offset offset) {
     canvas.drawRect(
       Rect.fromLTWH(
         offset.dx,
-        offset.dy + contentSection.size.height,
+        offset.dy + contentSection!.size.height,
         size.width,
         _dividerThickness,
       ),
@@ -734,15 +735,15 @@ class _RenderCupertinoAlert extends RenderBox {
   }
 
   @override
-  bool hitTestChildren(BoxHitTestResult result, { Offset position }) {
-    final MultiChildLayoutParentData contentSectionParentData = contentSection.parentData as MultiChildLayoutParentData;
-    final MultiChildLayoutParentData actionsSectionParentData = actionsSection.parentData as MultiChildLayoutParentData;
+  bool hitTestChildren(BoxHitTestResult result, { required Offset position }) {
+    final MultiChildLayoutParentData contentSectionParentData = contentSection!.parentData as MultiChildLayoutParentData;
+    final MultiChildLayoutParentData actionsSectionParentData = actionsSection!.parentData as MultiChildLayoutParentData;
     return result.addWithPaintOffset(
              offset: contentSectionParentData.offset,
              position: position,
              hitTest: (BoxHitTestResult result, Offset transformed) {
                assert(transformed == position - contentSectionParentData.offset);
-               return contentSection.hitTest(result, position: transformed);
+               return contentSection!.hitTest(result, position: transformed);
              },
            )
         || result.addWithPaintOffset(
@@ -750,7 +751,7 @@ class _RenderCupertinoAlert extends RenderBox {
              position: position,
              hitTest: (BoxHitTestResult result, Offset transformed) {
                assert(transformed == position - actionsSectionParentData.offset);
-               return actionsSection.hitTest(result, position: transformed);
+               return actionsSection!.hitTest(result, position: transformed);
              },
            );
   }
@@ -770,7 +771,7 @@ enum _AlertSections {
 // a SingleChildScrollView with a zero-sized Container.
 class _CupertinoAlertContentSection extends StatelessWidget {
   const _CupertinoAlertContentSection({
-    Key key,
+    Key? key,
     this.title,
     this.message,
     this.scrollController,
@@ -780,20 +781,20 @@ class _CupertinoAlertContentSection extends StatelessWidget {
   // the font of the title is bold.
   //
   // Typically a Text widget.
-  final Widget title;
+  final Widget? title;
 
   // An optional descriptive message that provides more details about the
   // reason for the alert.
   //
   // Typically a Text widget.
-  final Widget message;
+  final Widget? message;
 
   // A scroll controller that can be used to control the scrolling of the
   // content in the action sheet.
   //
   // Defaults to null, and is typically not needed, since most alert contents
   // are short.
-  final ScrollController scrollController;
+  final ScrollController? scrollController;
 
   @override
   Widget build(BuildContext context) {
@@ -811,7 +812,7 @@ class _CupertinoAlertContentSection extends StatelessWidget {
           style: message == null ? _kActionSheetContentStyle
               : _kActionSheetContentStyle.copyWith(fontWeight: FontWeight.w600),
           textAlign: TextAlign.center,
-          child: title,
+          child: title!,
         ),
       ));
     }
@@ -829,7 +830,7 @@ class _CupertinoAlertContentSection extends StatelessWidget {
             style: title == null ? _kActionSheetContentStyle.copyWith(fontWeight: FontWeight.w600)
                 : _kActionSheetContentStyle,
             textAlign: TextAlign.center,
-            child: message,
+            child: message!,
           ),
         ),
       );
@@ -869,8 +870,8 @@ class _CupertinoAlertContentSection extends StatelessWidget {
 // and layout.
 class _CupertinoAlertActionSection extends StatefulWidget {
   const _CupertinoAlertActionSection({
-    Key key,
-    @required this.children,
+    Key? key,
+    required this.children,
     this.scrollController,
     this.hasCancelButton,
   }) : assert(children != null),
@@ -883,9 +884,9 @@ class _CupertinoAlertActionSection extends StatefulWidget {
   //
   // Defaults to null, and is typically not needed, since most alerts
   // don't have many actions.
-  final ScrollController scrollController;
+  final ScrollController? scrollController;
 
-  final bool hasCancelButton;
+  final bool? hasCancelButton;
 
   @override
   _CupertinoAlertActionSectionState createState() => _CupertinoAlertActionSectionState();
@@ -894,7 +895,7 @@ class _CupertinoAlertActionSection extends StatefulWidget {
 class _CupertinoAlertActionSectionState extends State<_CupertinoAlertActionSection> {
   @override
   Widget build(BuildContext context) {
-    final double devicePixelRatio = MediaQuery.of(context).devicePixelRatio;
+    final double devicePixelRatio = MediaQuery.of(context)!.devicePixelRatio;
 
     final List<Widget> interactiveButtons = <Widget>[];
     for (int i = 0; i < widget.children.length; i += 1) {
@@ -911,7 +912,7 @@ class _CupertinoAlertActionSectionState extends State<_CupertinoAlertActionSecti
         child: _CupertinoAlertActionsRenderWidget(
           actionButtons: interactiveButtons,
           dividerThickness: _kDividerThickness / devicePixelRatio,
-          hasCancelButton: widget.hasCancelButton,
+          hasCancelButton: widget.hasCancelButton ?? false,
         ),
       ),
     );
@@ -925,7 +926,7 @@ class _CupertinoAlertActionSectionState extends State<_CupertinoAlertActionSecti
 // appropriately by _RenderCupertinoAlertActions.
 class _PressableActionButton extends StatefulWidget {
   const _PressableActionButton({
-    @required this.child,
+    required this.child,
   });
 
   final Widget child;
@@ -966,9 +967,9 @@ class _PressableActionButtonState extends State<_PressableActionButton> {
 // incoming isPressed property.
 class _ActionButtonParentDataWidget extends ParentDataWidget<_ActionButtonParentData> {
   const _ActionButtonParentDataWidget({
-    Key key,
-    this.isPressed,
-    @required Widget child,
+    Key? key,
+    required this.isPressed,
+    required Widget child,
   }) : super(key: key, child: child);
 
   final bool isPressed;
@@ -981,7 +982,7 @@ class _ActionButtonParentDataWidget extends ParentDataWidget<_ActionButtonParent
       parentData.isPressed = isPressed;
 
       // Force a repaint.
-      final AbstractNode targetParent = renderObject.parent;
+      final AbstractNode? targetParent = renderObject.parent;
       if (targetParent is RenderObject)
         targetParent.markNeedsPaint();
     }
@@ -1006,8 +1007,8 @@ class _ActionButtonParentData extends MultiChildLayoutParentData {
 // See _RenderCupertinoAlertActions for specific layout policy details.
 class _CupertinoAlertActionsRenderWidget extends MultiChildRenderObjectWidget {
   _CupertinoAlertActionsRenderWidget({
-    Key key,
-    @required List<Widget> actionButtons,
+    Key? key,
+    required List<Widget> actionButtons,
     double dividerThickness = 0.0,
     bool hasCancelButton = false,
   }) : _dividerThickness = dividerThickness,
@@ -1021,10 +1022,10 @@ class _CupertinoAlertActionsRenderWidget extends MultiChildRenderObjectWidget {
   RenderObject createRenderObject(BuildContext context) {
     return _RenderCupertinoAlertActions(
       dividerThickness: _dividerThickness,
-      dividerColor: CupertinoDynamicColor.resolve(_kButtonDividerColor, context),
+      dividerColor: CupertinoDynamicColor.resolve(_kButtonDividerColor, context)!,
       hasCancelButton: _hasCancelButton,
-      backgroundColor: CupertinoDynamicColor.resolve(_kBackgroundColor, context),
-      pressedColor: CupertinoDynamicColor.resolve(_kPressedColor, context),
+      backgroundColor: CupertinoDynamicColor.resolve(_kBackgroundColor, context)!,
+      pressedColor: CupertinoDynamicColor.resolve(_kPressedColor, context)!,
     );
   }
 
@@ -1032,10 +1033,10 @@ class _CupertinoAlertActionsRenderWidget extends MultiChildRenderObjectWidget {
   void updateRenderObject(BuildContext context, _RenderCupertinoAlertActions renderObject) {
     renderObject
       ..dividerThickness = _dividerThickness
-      ..dividerColor = CupertinoDynamicColor.resolve(_kButtonDividerColor, context)
+      ..dividerColor = CupertinoDynamicColor.resolve(_kButtonDividerColor, context)!
       ..hasCancelButton = _hasCancelButton
-      ..backgroundColor = CupertinoDynamicColor.resolve(_kBackgroundColor, context)
-      ..pressedColor = CupertinoDynamicColor.resolve(_kPressedColor, context);
+      ..backgroundColor = CupertinoDynamicColor.resolve(_kBackgroundColor, context)!
+      ..pressedColor = CupertinoDynamicColor.resolve(_kPressedColor, context)!;
   }
 }
 
@@ -1061,12 +1062,12 @@ class _RenderCupertinoAlertActions extends RenderBox
     with ContainerRenderObjectMixin<RenderBox, MultiChildLayoutParentData>,
         RenderBoxContainerDefaultsMixin<RenderBox, MultiChildLayoutParentData> {
   _RenderCupertinoAlertActions({
-    List<RenderBox> children,
+    List<RenderBox>? children,
     double dividerThickness = 0.0,
-    @required Color dividerColor,
+    required Color dividerColor,
     bool hasCancelButton = false,
-    Color backgroundColor,
-    Color pressedColor,
+    required Color backgroundColor,
+    required Color pressedColor,
   }) : _dividerThickness = dividerThickness,
        _hasCancelButton = hasCancelButton,
        _buttonBackgroundPaint = Paint()
@@ -1159,7 +1160,7 @@ class _RenderCupertinoAlertActions extends RenderBox
     if (childCount == 0)
       return 0.0;
     if (childCount == 1)
-      return firstChild.computeMaxIntrinsicHeight(width) + dividerThickness;
+      return firstChild!.computeMaxIntrinsicHeight(width) + dividerThickness;
     if (hasCancelButton && childCount < 4)
       return _computeMinIntrinsicHeightWithCancel(width);
     return _computeMinIntrinsicHeightWithoutCancel(width);
@@ -1170,13 +1171,13 @@ class _RenderCupertinoAlertActions extends RenderBox
   double _computeMinIntrinsicHeightWithCancel(double width) {
     assert(childCount == 2 || childCount == 3);
     if (childCount == 2) {
-      return firstChild.getMinIntrinsicHeight(width)
-        + childAfter(firstChild).getMinIntrinsicHeight(width)
+      return firstChild!.getMinIntrinsicHeight(width)
+        + childAfter(firstChild!)!.getMinIntrinsicHeight(width)
         + dividerThickness;
     }
-    return firstChild.getMinIntrinsicHeight(width)
-      + childAfter(firstChild).getMinIntrinsicHeight(width)
-      + childAfter(childAfter(firstChild)).getMinIntrinsicHeight(width)
+    return firstChild!.getMinIntrinsicHeight(width)
+      + childAfter(firstChild!)!.getMinIntrinsicHeight(width)
+      + childAfter(childAfter(firstChild!)!)!.getMinIntrinsicHeight(width)
       + (dividerThickness * 2);
   }
 
@@ -1185,9 +1186,9 @@ class _RenderCupertinoAlertActions extends RenderBox
   // + 50% the height of the 2nd button + 2 dividers.
   double _computeMinIntrinsicHeightWithoutCancel(double width) {
     assert(childCount >= 2);
-    return firstChild.getMinIntrinsicHeight(width)
+    return firstChild!.getMinIntrinsicHeight(width)
       + dividerThickness
-      + (0.5 * childAfter(firstChild).getMinIntrinsicHeight(width));
+      + (0.5 * childAfter(firstChild!)!.getMinIntrinsicHeight(width));
   }
 
   @override
@@ -1195,7 +1196,7 @@ class _RenderCupertinoAlertActions extends RenderBox
     if (childCount == 0)
       return 0.0;
     if (childCount == 1)
-      return firstChild.computeMaxIntrinsicHeight(width) + dividerThickness;
+      return firstChild!.computeMaxIntrinsicHeight(width) + dividerThickness;
     return _computeMaxIntrinsicHeightStacked(width);
   }
 
@@ -1206,7 +1207,7 @@ class _RenderCupertinoAlertActions extends RenderBox
 
     final double allDividersHeight = (childCount - 1) * dividerThickness;
     double heightAccumulation = allDividersHeight;
-    RenderBox button = firstChild;
+    RenderBox? button = firstChild;
     while (button != null) {
       heightAccumulation += button.getMaxIntrinsicHeight(width);
       button = childAfter(button);
@@ -1221,7 +1222,7 @@ class _RenderCupertinoAlertActions extends RenderBox
       maxHeight: double.infinity,
     );
 
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     int index = 0;
     double verticalOffset = 0.0;
     while (child != null) {
@@ -1269,8 +1270,8 @@ class _RenderCupertinoAlertActions extends RenderBox
 
     Offset accumulatingOffset = offset;
 
-    RenderBox child = firstChild;
-    RenderBox prevChild;
+    RenderBox? child = firstChild;
+    RenderBox? prevChild;
     while (child != null) {
       assert(child.parentData is _ActionButtonParentData);
       final _ActionButtonParentData currentButtonParentData = child.parentData as _ActionButtonParentData;
@@ -1327,7 +1328,7 @@ class _RenderCupertinoAlertActions extends RenderBox
   }
 
   void _drawButtons(PaintingContext context, Offset offset) {
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     while (child != null) {
       final MultiChildLayoutParentData childParentData = child.parentData as MultiChildLayoutParentData;
       context.paintChild(child, childParentData.offset + offset);
@@ -1336,7 +1337,7 @@ class _RenderCupertinoAlertActions extends RenderBox
   }
 
   @override
-  bool hitTestChildren(BoxHitTestResult result, { Offset position }) {
+  bool hitTestChildren(BoxHitTestResult result, { required Offset position }) {
     return defaultHitTestChildren(result, position: position);
   }
 }

packages/flutter/lib/src/cupertino/app.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/widgets.dart';
 
@@ -70,16 +68,16 @@ class CupertinoApp extends StatefulWidget {
   ///
   /// The boolean arguments, [routes], and [navigatorObservers], must not be null.
   const CupertinoApp({
-    Key key,
+    Key? key,
     this.navigatorKey,
     this.home,
     this.theme,
-    this.routes = const <String, WidgetBuilder>{},
+    Map<String, Widget Function(BuildContext)> this.routes = const <String, WidgetBuilder>{},
     this.initialRoute,
     this.onGenerateRoute,
     this.onGenerateInitialRoutes,
     this.onUnknownRoute,
-    this.navigatorObservers = const <NavigatorObserver>[],
+    List<NavigatorObserver> this.navigatorObservers = const <NavigatorObserver>[],
     this.builder,
     this.title = '',
     this.onGenerateTitle,
@@ -112,10 +110,10 @@ class CupertinoApp extends StatefulWidget {
 
   /// Creates a [CupertinoApp] that uses the [Router] instead of a [Navigator].
   const CupertinoApp.router({
-    Key key,
+    Key? key,
     this.routeInformationProvider,
-    @required this.routeInformationParser,
-    @required this.routerDelegate,
+    required RouteInformationParser<Object> this.routeInformationParser,
+    required RouterDelegate<Object> this.routerDelegate,
     this.backButtonDispatcher,
     this.theme,
     this.builder,
@@ -151,16 +149,16 @@ class CupertinoApp extends StatefulWidget {
        super(key: key);
 
   /// {@macro flutter.widgets.widgetsApp.navigatorKey}
-  final GlobalKey<NavigatorState> navigatorKey;
+  final GlobalKey<NavigatorState>? navigatorKey;
 
   /// {@macro flutter.widgets.widgetsApp.home}
-  final Widget home;
+  final Widget? home;
 
   /// The top-level [CupertinoTheme] styling.
   ///
   /// A null [theme] or unspecified [theme] attributes will default to iOS
   /// system values.
-  final CupertinoThemeData theme;
+  final CupertinoThemeData? theme;
 
   /// The application's top-level routing table.
   ///
@@ -170,37 +168,37 @@ class CupertinoApp extends StatefulWidget {
   /// an appropriate transition, including [Hero] animations, to the new route.
   ///
   /// {@macro flutter.widgets.widgetsApp.routes}
-  final Map<String, WidgetBuilder> routes;
+  final Map<String, WidgetBuilder>? routes;
 
   /// {@macro flutter.widgets.widgetsApp.initialRoute}
-  final String initialRoute;
+  final String? initialRoute;
 
   /// {@macro flutter.widgets.widgetsApp.onGenerateRoute}
-  final RouteFactory onGenerateRoute;
+  final RouteFactory? onGenerateRoute;
 
   /// {@macro flutter.widgets.widgetsApp.onGenerateInitialRoutes}
-  final InitialRouteListFactory onGenerateInitialRoutes;
+  final InitialRouteListFactory? onGenerateInitialRoutes;
 
   /// {@macro flutter.widgets.widgetsApp.onUnknownRoute}
-  final RouteFactory onUnknownRoute;
+  final RouteFactory? onUnknownRoute;
 
   /// {@macro flutter.widgets.widgetsApp.navigatorObservers}
-  final List<NavigatorObserver> navigatorObservers;
+  final List<NavigatorObserver>? navigatorObservers;
 
   /// {@macro flutter.widgets.widgetsApp.routeInformationProvider}
-  final RouteInformationProvider routeInformationProvider;
+  final RouteInformationProvider? routeInformationProvider;
 
   /// {@macro flutter.widgets.widgetsApp.routeInformationParser}
-  final RouteInformationParser<Object> routeInformationParser;
+  final RouteInformationParser<Object>? routeInformationParser;
 
   /// {@macro flutter.widgets.widgetsApp.routerDelegate}
-  final RouterDelegate<Object> routerDelegate;
+  final RouterDelegate<Object>? routerDelegate;
 
   /// {@macro flutter.widgets.widgetsApp.backButtonDispatcher}
-  final BackButtonDispatcher backButtonDispatcher;
+  final BackButtonDispatcher? backButtonDispatcher;
 
   /// {@macro flutter.widgets.widgetsApp.builder}
-  final TransitionBuilder builder;
+  final TransitionBuilder? builder;
 
   /// {@macro flutter.widgets.widgetsApp.title}
   ///
@@ -210,26 +208,26 @@ class CupertinoApp extends StatefulWidget {
   /// {@macro flutter.widgets.widgetsApp.onGenerateTitle}
   ///
   /// This value is passed unmodified to [WidgetsApp.onGenerateTitle].
-  final GenerateAppTitle onGenerateTitle;
+  final GenerateAppTitle? onGenerateTitle;
 
   /// {@macro flutter.widgets.widgetsApp.color}
-  final Color color;
+  final Color? color;
 
   /// {@macro flutter.widgets.widgetsApp.locale}
-  final Locale locale;
+  final Locale? locale;
 
   /// {@macro flutter.widgets.widgetsApp.localizationsDelegates}
-  final Iterable<LocalizationsDelegate<dynamic>> localizationsDelegates;
+  final Iterable<LocalizationsDelegate<dynamic>>? localizationsDelegates;
 
   /// {@macro flutter.widgets.widgetsApp.localeListResolutionCallback}
   ///
   /// This callback is passed along to the [WidgetsApp] built by this widget.
-  final LocaleListResolutionCallback localeListResolutionCallback;
+  final LocaleListResolutionCallback? localeListResolutionCallback;
 
   /// {@macro flutter.widgets.widgetsApp.localeResolutionCallback}
   ///
   /// This callback is passed along to the [WidgetsApp] built by this widget.
-  final LocaleResolutionCallback localeResolutionCallback;
+  final LocaleResolutionCallback? localeResolutionCallback;
 
   /// {@macro flutter.widgets.widgetsApp.supportedLocales}
   ///
@@ -282,7 +280,7 @@ class CupertinoApp extends StatefulWidget {
   /// ```
   /// {@end-tool}
   /// {@macro flutter.widgets.widgetsApp.shortcuts.seeAlso}
-  final Map<LogicalKeySet, Intent> shortcuts;
+  final Map<LogicalKeySet, Intent>? shortcuts;
 
   /// {@macro flutter.widgets.widgetsApp.actions}
   /// {@tool snippet}
@@ -315,7 +313,7 @@ class CupertinoApp extends StatefulWidget {
   /// ```
   /// {@end-tool}
   /// {@macro flutter.widgets.widgetsApp.actions.seeAlso}
-  final Map<Type, Action<Intent>> actions;
+  final Map<Type, Action<Intent>>? actions;
 
   @override
   _CupertinoAppState createState() => _CupertinoAppState();
@@ -341,7 +339,7 @@ class _AlwaysCupertinoScrollBehavior extends ScrollBehavior {
 }
 
 class _CupertinoAppState extends State<CupertinoApp> {
-  HeroController _heroController;
+  late HeroController _heroController;
   bool get _usesRouter => widget.routerDelegate != null;
 
   @override
@@ -357,7 +355,7 @@ class _CupertinoAppState extends State<CupertinoApp> {
   // _CupertinoLocalizationsDelegate.
   Iterable<LocalizationsDelegate<dynamic>> get _localizationsDelegates sync* {
     if (widget.localizationsDelegates != null)
-      yield* widget.localizationsDelegates;
+      yield* widget.localizationsDelegates!;
     yield DefaultCupertinoLocalizations.delegate;
   }
 
@@ -375,14 +373,14 @@ class _CupertinoAppState extends State<CupertinoApp> {
 
   WidgetsApp _buildWidgetApp(BuildContext context) {
     final CupertinoThemeData effectiveThemeData = CupertinoTheme.of(context);
-    final Color color = CupertinoDynamicColor.resolve(widget.color ?? effectiveThemeData.primaryColor, context);
+    final Color color = CupertinoDynamicColor.resolve(widget.color ?? effectiveThemeData.primaryColor, context)!;
 
     if (_usesRouter) {
       return WidgetsApp.router(
         key: GlobalObjectKey(this),
         routeInformationProvider: widget.routeInformationProvider,
-        routeInformationParser: widget.routeInformationParser,
-        routerDelegate: widget.routerDelegate,
+        routeInformationParser: widget.routeInformationParser!,
+        routerDelegate: widget.routerDelegate!,
         backButtonDispatcher: widget.backButtonDispatcher,
         builder: widget.builder,
         title: widget.title,
@@ -407,12 +405,12 @@ class _CupertinoAppState extends State<CupertinoApp> {
     return WidgetsApp(
       key: GlobalObjectKey(this),
       navigatorKey: widget.navigatorKey,
-      navigatorObservers: widget.navigatorObservers,
+      navigatorObservers: widget.navigatorObservers!,
       pageRouteBuilder: <T>(RouteSettings settings, WidgetBuilder builder) {
         return CupertinoPageRoute<T>(settings: settings, builder: builder);
       },
       home: widget.home,
-      routes: widget.routes,
+      routes: widget.routes!,
       initialRoute: widget.initialRoute,
       onGenerateRoute: widget.onGenerateRoute,
       onGenerateInitialRoutes: widget.onGenerateInitialRoutes,

packages/flutter/lib/src/cupertino/bottom_tab_bar.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 ImageFilter;
 
 import 'package:flutter/widgets.dart';
@@ -53,8 +51,8 @@ const Color _kDefaultTabBarInactiveColor = CupertinoColors.inactiveGray;
 class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
   /// Creates a tab bar in the iOS style.
   const CupertinoTabBar({
-    Key key,
-    @required this.items,
+    Key? key,
+    required this.items,
     this.onTap,
     this.currentIndex = 0,
     this.backgroundColor,
@@ -89,7 +87,7 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
   /// The widget creating the bottom navigation bar needs to keep track of the
   /// current index and call `setState` to rebuild it with the newly provided
   /// index.
-  final ValueChanged<int> onTap;
+  final ValueChanged<int>? onTap;
 
   /// The index into [items] of the current active item.
   ///
@@ -102,13 +100,13 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
   /// behind it.
   ///
   /// Defaults to [CupertinoTheme]'s `barBackgroundColor` when null.
-  final Color backgroundColor;
+  final Color? backgroundColor;
 
   /// The foreground color of the icon and title for the [BottomNavigationBarItem]
   /// of the selected tab.
   ///
   /// Defaults to [CupertinoTheme]'s `primaryColor` if null.
-  final Color activeColor;
+  final Color? activeColor;
 
   /// The foreground color of the icon and title for the [BottomNavigationBarItem]s
   /// in the unselected state.
@@ -129,7 +127,7 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
   /// The border of the [CupertinoTabBar].
   ///
   /// The default value is a one physical pixel top border with grey color.
-  final Border border;
+  final Border? border;
 
   @override
   Size get preferredSize => const Size.fromHeight(_kTabBarHeight);
@@ -139,14 +137,15 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
   bool opaque(BuildContext context) {
     final Color backgroundColor =
         this.backgroundColor ?? CupertinoTheme.of(context).barBackgroundColor;
-    return CupertinoDynamicColor.resolve(backgroundColor, context).alpha == 0xFF;
+    return CupertinoDynamicColor.resolve(backgroundColor, context)!.alpha == 0xFF;
   }
 
   @override
   Widget build(BuildContext context) {
-    final double bottomPadding = MediaQuery.of(context).padding.bottom;
+    assert(debugCheckHasMediaQuery(context));
+    final double bottomPadding = MediaQuery.of(context)!.padding.bottom;
 
-    final Color backgroundColor = CupertinoDynamicColor.resolve(
+    final Color? backgroundColor = CupertinoDynamicColor.resolve(
       this.backgroundColor ?? CupertinoTheme.of(context).barBackgroundColor,
       context,
     );
@@ -158,16 +157,16 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
     }
 
     // Return the border as is when it's a subclass.
-    final Border resolvedBorder = border == null || border.runtimeType != Border
+    final Border? resolvedBorder = border == null || border.runtimeType != Border
       ? border
       : Border(
-        top: resolveBorderSide(border.top),
-        left: resolveBorderSide(border.left),
-        bottom: resolveBorderSide(border.bottom),
-        right: resolveBorderSide(border.right),
+        top: resolveBorderSide(border!.top),
+        left: resolveBorderSide(border!.left),
+        bottom: resolveBorderSide(border!.bottom),
+        right: resolveBorderSide(border!.right),
       );
 
-    final Color inactive = CupertinoDynamicColor.resolve(inactiveColor, context);
+    final Color? inactive = CupertinoDynamicColor.resolve(inactiveColor, context);
     Widget result = DecoratedBox(
       decoration: BoxDecoration(
         border: resolvedBorder,
@@ -210,7 +209,7 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
 
   List<Widget> _buildTabItems(BuildContext context) {
     final List<Widget> result = <Widget>[];
-    final CupertinoLocalizations localizations = CupertinoLocalizations.of(context);
+    final CupertinoLocalizations? localizations = CupertinoLocalizations.of(context);
     assert(
       localizations != null,
       'CupertinoTabBar requires a Localizations parent in order to provide an '
@@ -227,13 +226,13 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
           Expanded(
             child: Semantics(
               selected: active,
-              hint: localizations.tabSemanticsLabel(
+              hint: localizations!.tabSemanticsLabel(
                 tabIndex: index + 1,
                 tabCount: items.length,
               ),
               child: GestureDetector(
                 behavior: HitTestBehavior.opaque,
-                onTap: onTap == null ? null : () { onTap(index); },
+                onTap: onTap == null ? null : () { onTap!(index); },
                 child: Padding(
                   padding: const EdgeInsets.only(bottom: 4.0),
                   child: Column(
@@ -257,17 +256,17 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
       Expanded(
         child: Center(child: active ? item.activeIcon : item.icon),
       ),
-      if (item.title != null) item.title,
-      if (item.label != null) Text(item.label),
+      if (item.title != null) item.title!,
+      if (item.label != null) Text(item.label!),
     ];
   }
 
   /// Change the active tab item's icon and title colors to active.
-  Widget _wrapActiveItem(BuildContext context, Widget item, { @required bool active }) {
+  Widget _wrapActiveItem(BuildContext context, Widget item, { required bool active }) {
     if (!active)
       return item;
 
-    final Color activeColor = CupertinoDynamicColor.resolve(
+    final Color? activeColor = CupertinoDynamicColor.resolve(
       this.activeColor ?? CupertinoTheme.of(context).primaryColor,
       context,
     );
@@ -283,15 +282,15 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
   /// Create a clone of the current [CupertinoTabBar] but with provided
   /// parameters overridden.
   CupertinoTabBar copyWith({
-    Key key,
-    List<BottomNavigationBarItem> items,
-    Color backgroundColor,
-    Color activeColor,
-    Color inactiveColor,
-    double iconSize,
-    Border border,
-    int currentIndex,
-    ValueChanged<int> onTap,
+    Key? key,
+    List<BottomNavigationBarItem>? items,
+    Color? backgroundColor,
+    Color? activeColor,
+    Color? inactiveColor,
+    double? iconSize,
+    Border? border,
+    int? currentIndex,
+    ValueChanged<int>? onTap,
   }) {
     return CupertinoTabBar(
       key: key ?? this.key,

packages/flutter/lib/src/cupertino/context_menu.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 'dart:ui' as ui;
 import 'package:flutter/gestures.dart' show kMinFlingVelocity, kLongPressTimeout;
@@ -42,7 +40,7 @@ typedef _ContextMenuPreviewBuilderChildless = Widget Function(
 // paintBounds in global coordinates.
 Rect _getRect(GlobalKey globalKey) {
   assert(globalKey.currentContext != null);
-  final RenderBox renderBoxContainer = globalKey.currentContext.findRenderObject() as RenderBox;
+  final RenderBox renderBoxContainer = globalKey.currentContext!.findRenderObject() as RenderBox;
   final Offset containerOffset = renderBoxContainer.localToGlobal(
     renderBoxContainer.paintBounds.topLeft,
   );
@@ -129,9 +127,9 @@ class CupertinoContextMenu extends StatefulWidget {
   ///
   /// [child] is required and cannot be null.
   CupertinoContextMenu({
-    Key key,
-    @required this.actions,
-    @required this.child,
+    Key? key,
+    required this.actions,
+    required this.child,
     this.previewBuilder,
   }) : assert(actions != null && actions.isNotEmpty),
        assert(child != null),
@@ -220,7 +218,7 @@ class CupertinoContextMenu extends StatefulWidget {
   /// ```
   ///
   /// {@end-tool}
-  final ContextMenuPreviewBuilder previewBuilder;
+  final ContextMenuPreviewBuilder? previewBuilder;
 
   @override
   _CupertinoContextMenuState createState() => _CupertinoContextMenuState();
@@ -230,10 +228,10 @@ class _CupertinoContextMenuState extends State<CupertinoContextMenu> with Ticker
   final GlobalKey _childGlobalKey = GlobalKey();
   bool _childHidden = false;
   // Animates the child while it's opening.
-  AnimationController _openController;
-  Rect _decoyChildEndRect;
-  OverlayEntry _lastOverlayEntry;
-  _ContextMenuRoute<void> _route;
+  late AnimationController _openController;
+  Rect? _decoyChildEndRect;
+  OverlayEntry? _lastOverlayEntry;
+  _ContextMenuRoute<void>? _route;
 
   @override
   void initState() {
@@ -255,7 +253,7 @@ class _CupertinoContextMenuState extends State<CupertinoContextMenu> with Ticker
   // it.
   _ContextMenuLocation get _contextMenuLocation {
     final Rect childRect = _getRect(_childGlobalKey);
-    final double screenWidth = MediaQuery.of(context).size.width;
+    final double screenWidth = MediaQuery.of(context)!.size.width;
 
     final double center = screenWidth / 2;
     final bool centerDividesChild = childRect.left < center
@@ -286,16 +284,16 @@ class _CupertinoContextMenuState extends State<CupertinoContextMenu> with Ticker
         sigmaY: 5.0,
       ),
       contextMenuLocation: _contextMenuLocation,
-      previousChildRect: _decoyChildEndRect,
+      previousChildRect: _decoyChildEndRect!,
       builder: (BuildContext context, Animation<double> animation) {
         if (widget.previewBuilder == null) {
           return widget.child;
         }
-        return widget.previewBuilder(context, animation, widget.child);
+        return widget.previewBuilder!(context, animation, widget.child);
       },
     );
-    Navigator.of(context, rootNavigator: true).push<void>(_route);
-    _route.animation.addStatusListener(_routeAnimationStatusListener);
+    Navigator.of(context, rootNavigator: true)!.push<void>(_route!);
+    _route!.animation!.addStatusListener(_routeAnimationStatusListener);
   }
 
   void _onDecoyAnimationStatusChange(AnimationStatus animationStatus) {
@@ -319,7 +317,7 @@ class _CupertinoContextMenuState extends State<CupertinoContextMenu> with Ticker
         // because _ContextMenuRoute renders its first frame offscreen.
         // Otherwise there would be a visible flash when nothing is rendered for
         // one frame.
-        SchedulerBinding.instance.addPostFrameCallback((Duration _) {
+        SchedulerBinding.instance!.addPostFrameCallback((Duration _) {
           _lastOverlayEntry?.remove();
           _lastOverlayEntry = null;
           _openController.reset();
@@ -340,7 +338,7 @@ class _CupertinoContextMenuState extends State<CupertinoContextMenu> with Ticker
     setState(() {
       _childHidden = false;
     });
-    _route.animation.removeStatusListener(_routeAnimationStatusListener);
+    _route!.animation!.removeStatusListener(_routeAnimationStatusListener);
     _route = null;
   }
 
@@ -393,7 +391,7 @@ class _CupertinoContextMenuState extends State<CupertinoContextMenu> with Ticker
         );
       },
     );
-    Overlay.of(context).insert(_lastOverlayEntry);
+    Overlay.of(context)!.insert(_lastOverlayEntry!);
     _openController.forward();
   }
 
@@ -431,17 +429,17 @@ class _CupertinoContextMenuState extends State<CupertinoContextMenu> with Ticker
 // siblings of the original child.
 class _DecoyChild extends StatefulWidget {
   const _DecoyChild({
-    Key key,
+    Key? key,
     this.beginRect,
-    this.controller,
+    required this.controller,
     this.endRect,
     this.child,
   }) : super(key: key);
 
-  final Rect beginRect;
+  final Rect? beginRect;
   final AnimationController controller;
-  final Rect endRect;
-  final Widget child;
+  final Rect? endRect;
+  final Widget? child;
 
   @override
   _DecoyChildState createState() => _DecoyChildState();
@@ -454,8 +452,8 @@ class _DecoyChildState extends State<_DecoyChild> with TickerProviderStateMixin
   static const Color _masklessColor = Color(0xFFFFFFFF);
 
   final GlobalKey _childGlobalKey = GlobalKey();
-  Animation<Color> _mask;
-  Animation<Rect> _rect;
+  late Animation<Color> _mask;
+  late Animation<Rect?> _rect;
 
   @override
   void initState() {
@@ -470,18 +468,18 @@ class _DecoyChildState extends State<_DecoyChild> with TickerProviderStateMixin
       intervalOff: 0.5,
     );
 
-    final Rect midRect =  widget.beginRect.deflate(
-      widget.beginRect.width * (_kOpenScale - 1.0) / 2,
+    final Rect midRect =  widget.beginRect!.deflate(
+      widget.beginRect!.width * (_kOpenScale - 1.0) / 2,
     );
-    _rect = TweenSequence<Rect>(<TweenSequenceItem<Rect>>[
-      TweenSequenceItem<Rect>(
+    _rect = TweenSequence<Rect?>(<TweenSequenceItem<Rect?>>[
+      TweenSequenceItem<Rect?>(
         tween: RectTween(
           begin: widget.beginRect,
           end: midRect,
         ).chain(CurveTween(curve: Curves.easeInOutCubic)),
         weight: 1.0,
       ),
-      TweenSequenceItem<Rect>(
+      TweenSequenceItem<Rect?>(
         tween: RectTween(
           begin: midRect,
           end: widget.endRect,
@@ -508,12 +506,12 @@ class _DecoyChildState extends State<_DecoyChild> with TickerProviderStateMixin
     super.dispose();
   }
 
-  Widget _buildAnimation(BuildContext context, Widget child) {
+  Widget _buildAnimation(BuildContext context, Widget? child) {
     final Color color = widget.controller.status == AnimationStatus.reverse
       ? _masklessColor
       : _mask.value;
     return Positioned.fromRect(
-      rect: _rect.value,
+      rect: _rect.value!,
       // TODO(justinmc): When ShaderMask is supported on web, remove this
       // conditional and use ShaderMask everywhere.
       // https://github.com/flutter/flutter/issues/52967.
@@ -550,13 +548,13 @@ class _DecoyChildState extends State<_DecoyChild> with TickerProviderStateMixin
 class _ContextMenuRoute<T> extends PopupRoute<T> {
   // Build a _ContextMenuRoute.
   _ContextMenuRoute({
-    @required List<Widget> actions,
-    @required _ContextMenuLocation contextMenuLocation,
+    required List<Widget> actions,
+    required _ContextMenuLocation contextMenuLocation,
     this.barrierLabel,
-    _ContextMenuPreviewBuilderChildless builder,
-    ui.ImageFilter filter,
-    Rect previousChildRect,
-    RouteSettings settings,
+    _ContextMenuPreviewBuilderChildless? builder,
+    ui.ImageFilter? filter,
+    required Rect previousChildRect,
+    RouteSettings? settings,
   }) : assert(actions != null && actions.isNotEmpty),
        assert(contextMenuLocation != null),
        _actions = actions,
@@ -576,15 +574,15 @@ class _ContextMenuRoute<T> extends PopupRoute<T> {
     Duration(milliseconds: 335);
 
   final List<Widget> _actions;
-  final _ContextMenuPreviewBuilderChildless _builder;
+  final _ContextMenuPreviewBuilderChildless? _builder;
   final GlobalKey _childGlobalKey = GlobalKey();
   final _ContextMenuLocation _contextMenuLocation;
   bool _externalOffstage = false;
   bool _internalOffstage = false;
-  Orientation _lastOrientation;
+  Orientation? _lastOrientation;
   // The Rect of the child at the moment that the CupertinoContextMenu opens.
   final Rect _previousChildRect;
-  double _scale = 1.0;
+  double? _scale = 1.0;
   final GlobalKey _sheetGlobalKey = GlobalKey();
 
   static final CurveTween _curve = CurveTween(
@@ -594,17 +592,17 @@ class _ContextMenuRoute<T> extends PopupRoute<T> {
     curve: Curves.easeInBack,
   );
   static final RectTween _rectTween = RectTween();
-  static final Animatable<Rect> _rectAnimatable = _rectTween.chain(_curve);
+  static final Animatable<Rect?> _rectAnimatable = _rectTween.chain(_curve);
   static final RectTween _rectTweenReverse = RectTween();
-  static final Animatable<Rect> _rectAnimatableReverse = _rectTweenReverse
+  static final Animatable<Rect?> _rectAnimatableReverse = _rectTweenReverse
     .chain(
       _curveReverse,
     );
   static final RectTween _sheetRectTween = RectTween();
-  final Animatable<Rect> _sheetRectAnimatable = _sheetRectTween.chain(
+  final Animatable<Rect?> _sheetRectAnimatable = _sheetRectTween.chain(
     _curve,
   );
-  final Animatable<Rect> _sheetRectAnimatableReverse = _sheetRectTween.chain(
+  final Animatable<Rect?> _sheetRectAnimatableReverse = _sheetRectTween.chain(
     _curveReverse,
   );
   static final Tween<double> _sheetScaleTween = Tween<double>();
@@ -617,10 +615,10 @@ class _ContextMenuRoute<T> extends PopupRoute<T> {
       _curveReverse,
     );
   final Tween<double> _opacityTween = Tween<double>(begin: 0.0, end: 1.0);
-  Animation<double> _sheetOpacity;
+  late Animation<double> _sheetOpacity;
 
   @override
-  final String barrierLabel;
+  final String? barrierLabel;
 
   @override
   Color get barrierColor => _kModalBarrierColor;
@@ -660,7 +658,7 @@ class _ContextMenuRoute<T> extends PopupRoute<T> {
   }
 
   // The place to start the sheetRect animation from.
-  static Rect _getSheetRectBegin(Orientation orientation, _ContextMenuLocation contextMenuLocation, Rect childRect, Rect sheetRect) {
+  static Rect _getSheetRectBegin(Orientation? orientation, _ContextMenuLocation contextMenuLocation, Rect childRect, Rect sheetRect) {
     switch (contextMenuLocation) {
       case _ContextMenuLocation.center:
         final Offset target = orientation == Orientation.portrait
@@ -685,10 +683,10 @@ class _ContextMenuRoute<T> extends PopupRoute<T> {
     _scale = scale;
     _opacityTween.end = opacity;
     _sheetOpacity = _opacityTween.animate(CurvedAnimation(
-      parent: animation,
+      parent: animation!,
       curve: const Interval(0.9, 1.0),
     ));
-    Navigator.of(context).pop();
+    Navigator.of(context)!.pop();
   }
 
   // Take measurements on the child and _ContextMenuSheet and update the
@@ -696,7 +694,7 @@ class _ContextMenuRoute<T> extends PopupRoute<T> {
   void _updateTweenRects() {
     final Rect childRect = _scale == null
       ? _getRect(_childGlobalKey)
-      : _getScaledRect(_childGlobalKey, _scale);
+      : _getScaledRect(_childGlobalKey, _scale!);
     _rectTween.begin = _previousChildRect;
     _rectTween.end = childRect;
 
@@ -751,7 +749,7 @@ class _ContextMenuRoute<T> extends PopupRoute<T> {
 
     // Render one frame offstage in the final position so that we can take
     // measurements of its layout and then animate to them.
-    SchedulerBinding.instance.addPostFrameCallback((Duration _) {
+    SchedulerBinding.instance!.addPostFrameCallback((Duration _) {
       _updateTweenRects();
       _internalOffstage = false;
       _setOffstageInternally();
@@ -775,8 +773,8 @@ class _ContextMenuRoute<T> extends PopupRoute<T> {
     // buildTransitions as child, the idea being that buildTransitions will
     // animate the entire page into the scene. In the case of _ContextMenuRoute,
     // two individual pieces of the page are animated into the scene in
-    // buildTransitions, and null is returned here.
-    return null;
+    // buildTransitions, and a Container is returned here.
+    return Container();
   }
 
   @override
@@ -790,11 +788,11 @@ class _ContextMenuRoute<T> extends PopupRoute<T> {
         if (!animation.isCompleted) {
           final bool reverse = animation.status == AnimationStatus.reverse;
           final Rect rect = reverse
-            ? _rectAnimatableReverse.evaluate(animation)
-            : _rectAnimatable.evaluate(animation);
+            ? _rectAnimatableReverse.evaluate(animation)!
+            : _rectAnimatable.evaluate(animation)!;
           final Rect sheetRect = reverse
-            ? _sheetRectAnimatableReverse.evaluate(animation)
-            : _sheetRectAnimatable.evaluate(animation);
+            ? _sheetRectAnimatableReverse.evaluate(animation)!
+            : _sheetRectAnimatable.evaluate(animation)!;
           final double sheetScale = reverse
             ? _sheetScaleAnimatableReverse.evaluate(animation)
             : _sheetScaleAnimatable.evaluate(animation);
@@ -819,7 +817,7 @@ class _ContextMenuRoute<T> extends PopupRoute<T> {
               Positioned.fromRect(
                 key: _childGlobalKey,
                 rect: rect,
-                child: _builder(context, animation),
+                child: _builder!(context, animation),
               ),
             ],
           );
@@ -829,7 +827,7 @@ class _ContextMenuRoute<T> extends PopupRoute<T> {
         // in the final position.
         return _ContextMenuRouteStatic(
           actions: _actions,
-          child: _builder(context, animation),
+          child: _builder!(context, animation),
           childGlobalKey: _childGlobalKey,
           contextMenuLocation: _contextMenuLocation,
           onDismiss: _onDismiss,
@@ -845,25 +843,25 @@ class _ContextMenuRoute<T> extends PopupRoute<T> {
 // animating out.
 class _ContextMenuRouteStatic extends StatefulWidget {
   const _ContextMenuRouteStatic({
-    Key key,
+    Key? key,
     this.actions,
-    @required this.child,
+    required this.child,
     this.childGlobalKey,
-    @required this.contextMenuLocation,
+    required this.contextMenuLocation,
     this.onDismiss,
-    @required this.orientation,
+    required this.orientation,
     this.sheetGlobalKey,
   }) : assert(contextMenuLocation != null),
        assert(orientation != null),
        super(key: key);
 
-  final List<Widget> actions;
+  final List<Widget>? actions;
   final Widget child;
-  final GlobalKey childGlobalKey;
+  final GlobalKey? childGlobalKey;
   final _ContextMenuLocation contextMenuLocation;
-  final _DismissCallback onDismiss;
+  final _DismissCallback? onDismiss;
   final Orientation orientation;
-  final GlobalKey sheetGlobalKey;
+  final GlobalKey? sheetGlobalKey;
 
   @override
   _ContextMenuRouteStaticState createState() => _ContextMenuRouteStaticState();
@@ -879,13 +877,13 @@ class _ContextMenuRouteStaticState extends State<_ContextMenuRouteStatic> with T
   static const double _kDamping = 400.0;
   static const Duration _kMoveControllerDuration = Duration(milliseconds: 600);
 
-  Offset _dragOffset;
+  late Offset _dragOffset;
   double _lastScale = 1.0;
-  AnimationController _moveController;
-  AnimationController _sheetController;
-  Animation<Offset> _moveAnimation;
-  Animation<double> _sheetScaleAnimation;
-  Animation<double> _sheetOpacityAnimation;
+  late AnimationController _moveController;
+  late AnimationController _sheetController;
+  late Animation<Offset> _moveAnimation;
+  late Animation<double> _sheetScaleAnimation;
+  late Animation<double> _sheetOpacityAnimation;
 
   // The scale of the child changes as a function of the distance it is dragged.
   static double _getScale(Orientation orientation, double maxDragDistance, double dy) {
@@ -934,7 +932,7 @@ class _ContextMenuRouteStaticState extends State<_ContextMenuRouteStatic> with T
 
     // Dismiss if the drag is enough to scale down all the way.
     if (_lastScale == _kMinScale) {
-      widget.onDismiss(context, _lastScale, _sheetOpacityAnimation.value);
+      widget.onDismiss!(context, _lastScale, _sheetOpacityAnimation.value);
       return;
     }
 
@@ -967,7 +965,7 @@ class _ContextMenuRouteStaticState extends State<_ContextMenuRouteStatic> with T
     if (_moveAnimation.value.dy == 0.0) {
       return;
     }
-    widget.onDismiss(context, _lastScale, _sheetOpacityAnimation.value);
+    widget.onDismiss!(context, _lastScale, _sheetOpacityAnimation.value);
   }
 
   Alignment _getChildAlignment(Orientation orientation, _ContextMenuLocation contextMenuLocation) {
@@ -998,7 +996,7 @@ class _ContextMenuRouteStaticState extends State<_ContextMenuRouteStatic> with T
       _moveAnimation = Tween<Offset>(
         begin: Offset.zero,
         end: Offset(
-          endX.clamp(-_kPadding, _kPadding) as double,
+          endX.clamp(-_kPadding, _kPadding),
           endY,
         ),
       ).animate(
@@ -1048,7 +1046,7 @@ class _ContextMenuRouteStaticState extends State<_ContextMenuRouteStatic> with T
         builder: _buildSheetAnimation,
         child: _ContextMenuSheet(
           key: widget.sheetGlobalKey,
-          actions: widget.actions,
+          actions: widget.actions!,
           contextMenuLocation: widget.contextMenuLocation,
           orientation: widget.orientation,
         ),
@@ -1068,7 +1066,7 @@ class _ContextMenuRouteStaticState extends State<_ContextMenuRouteStatic> with T
   }
 
   // Build the animation for the _ContextMenuSheet.
-  Widget _buildSheetAnimation(BuildContext context, Widget child) {
+  Widget _buildSheetAnimation(BuildContext context, Widget? child) {
     return Transform.scale(
       alignment: _ContextMenuRoute.getSheetAlignment(widget.contextMenuLocation),
       scale: _sheetScaleAnimation.value,
@@ -1080,10 +1078,10 @@ class _ContextMenuRouteStaticState extends State<_ContextMenuRouteStatic> with T
   }
 
   // Build the animation for the child.
-  Widget _buildChildAnimation(BuildContext context, Widget child) {
+  Widget _buildChildAnimation(BuildContext context, Widget? child) {
     _lastScale = _getScale(
       widget.orientation,
-      MediaQuery.of(context).size.height,
+      MediaQuery.of(context)!.size.height,
       _moveAnimation.value.dy,
     );
     return Transform.scale(
@@ -1094,7 +1092,7 @@ class _ContextMenuRouteStaticState extends State<_ContextMenuRouteStatic> with T
   }
 
   // Build the animation for the overall draggable dismissible content.
-  Widget _buildAnimation(BuildContext context, Widget child) {
+  Widget _buildAnimation(BuildContext context, Widget? child) {
     return Transform.translate(
       offset: _moveAnimation.value,
       child: child,
@@ -1178,10 +1176,10 @@ class _ContextMenuRouteStaticState extends State<_ContextMenuRouteStatic> with T
 // list of actions that are typically CupertinoContextMenuActions.
 class _ContextMenuSheet extends StatelessWidget {
   _ContextMenuSheet({
-    Key key,
-    @required this.actions,
-    @required _ContextMenuLocation contextMenuLocation,
-    @required Orientation orientation,
+    Key? key,
+    required this.actions,
+    required _ContextMenuLocation contextMenuLocation,
+    required Orientation orientation,
   }) : assert(actions != null && actions.isNotEmpty),
        assert(contextMenuLocation != null),
        assert(orientation != null),
@@ -1261,11 +1259,11 @@ class _ContextMenuSheet extends StatelessWidget {
 // offColor at the times given by intervalOn and intervalOff.
 class _OnOffAnimation<T> extends CompoundAnimation<T> {
   _OnOffAnimation({
-    AnimationController controller,
-    @required T onValue,
-    @required T offValue,
-    @required double intervalOn,
-    @required double intervalOff,
+    required AnimationController controller,
+    required T onValue,
+    required T offValue,
+    required double intervalOn,
+    required double intervalOff,
   }) : _offValue = offValue,
        assert(intervalOn >= 0.0 && intervalOn <= 1.0),
        assert(intervalOff >= 0.0 && intervalOff <= 1.0),

packages/flutter/lib/src/cupertino/context_menu_action.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/widgets.dart';
 import 'colors.dart';
@@ -16,8 +14,8 @@ import 'colors.dart';
 class CupertinoContextMenuAction extends StatefulWidget {
   /// Construct a CupertinoContextMenuAction.
   const CupertinoContextMenuAction({
-    Key key,
-    @required this.child,
+    Key? key,
+    required this.child,
     this.isDefaultAction = false,
     this.isDestructiveAction = false,
     this.onPressed,
@@ -39,13 +37,13 @@ class CupertinoContextMenuAction extends StatefulWidget {
   final bool isDestructiveAction;
 
   /// Called when the action is pressed.
-  final VoidCallback onPressed;
+  final VoidCallback? onPressed;
 
   /// An optional icon to display to the right of the child.
   ///
   /// Will be colored in the same way as the [TextStyle] used for [child] (for
   /// example, if using [isDestructiveAction]).
-  final IconData trailingIcon;
+  final IconData? trailingIcon;
 
   @override
   _CupertinoContextMenuActionState createState() => _CupertinoContextMenuActionState();

packages/flutter/lib/src/cupertino/date_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 'dart:math' as math;
 
 import 'package:flutter/scheduler.dart';
@@ -67,8 +65,8 @@ void _animateColumnControllerToItem(FixedExtentScrollController controller, int
 // column will be extended equally to the remaining width.
 class _DatePickerLayoutDelegate extends MultiChildLayoutDelegate {
   _DatePickerLayoutDelegate({
-    @required this.columnWidths,
-    @required this.textDirectionFactor,
+    required this.columnWidths,
+    required this.textDirectionFactor,
   }) : assert(columnWidths != null),
        assert(textDirectionFactor != null);
 
@@ -236,10 +234,10 @@ class CupertinoDatePicker extends StatefulWidget {
   ///
   /// [use24hFormat] decides whether 24 hour format is used. Defaults to false.
   CupertinoDatePicker({
-    Key key,
+    Key? key,
     this.mode = CupertinoDatePickerMode.dateAndTime,
-    @required this.onDateTimeChanged,
-    DateTime initialDateTime,
+    required this.onDateTimeChanged,
+    DateTime? initialDateTime,
     this.minimumDate,
     this.maximumDate,
     this.minimumYear = 1,
@@ -258,11 +256,11 @@ class CupertinoDatePicker extends StatefulWidget {
        super(key: key) {
     assert(this.initialDateTime != null);
     assert(
-      mode != CupertinoDatePickerMode.dateAndTime || minimumDate == null || !this.initialDateTime.isBefore(minimumDate),
+      mode != CupertinoDatePickerMode.dateAndTime || minimumDate == null || !this.initialDateTime.isBefore(minimumDate!),
       'initial date is before minimum date',
     );
     assert(
-      mode != CupertinoDatePickerMode.dateAndTime || maximumDate == null || !this.initialDateTime.isAfter(maximumDate),
+      mode != CupertinoDatePickerMode.dateAndTime || maximumDate == null || !this.initialDateTime.isAfter(maximumDate!),
       'initial date is after maximum date',
     );
     assert(
@@ -270,15 +268,15 @@ class CupertinoDatePicker extends StatefulWidget {
       'initial year is not greater than minimum year, or minimum year is not positive',
     );
     assert(
-      mode != CupertinoDatePickerMode.date || maximumYear == null || this.initialDateTime.year <= maximumYear,
+      mode != CupertinoDatePickerMode.date || maximumYear == null || this.initialDateTime.year <= maximumYear!,
       'initial year is not smaller than maximum year',
     );
     assert(
-      mode != CupertinoDatePickerMode.date || minimumDate == null || !minimumDate.isAfter(this.initialDateTime),
+      mode != CupertinoDatePickerMode.date || minimumDate == null || !minimumDate!.isAfter(this.initialDateTime),
       'initial date ${this.initialDateTime} is not greater than or equal to minimumDate $minimumDate',
     );
     assert(
-      mode != CupertinoDatePickerMode.date || maximumDate == null || !maximumDate.isBefore(this.initialDateTime),
+      mode != CupertinoDatePickerMode.date || maximumDate == null || !maximumDate!.isBefore(this.initialDateTime),
       'initial date ${this.initialDateTime} is not less than or equal to maximumDate $maximumDate',
     );
     assert(
@@ -313,7 +311,7 @@ class CupertinoDatePicker extends StatefulWidget {
   ///
   /// Defaults to null. When set to null, the picker does not impose a limit on
   /// the earliest [DateTime] the user can select.
-  final DateTime minimumDate;
+  final DateTime? minimumDate;
 
   /// The maximum selectable date that the picker can settle on.
   ///
@@ -328,7 +326,7 @@ class CupertinoDatePicker extends StatefulWidget {
   ///
   /// Defaults to null. When set to null, the picker does not impose a limit on
   /// the latest [DateTime] the user can select.
-  final DateTime maximumDate;
+  final DateTime? maximumDate;
 
   /// Minimum year that the picker can be scrolled to in
   /// [CupertinoDatePickerMode.date] mode. Defaults to 1 and must not be null.
@@ -336,7 +334,7 @@ class CupertinoDatePicker extends StatefulWidget {
 
   /// Maximum year that the picker can be scrolled to in
   /// [CupertinoDatePickerMode.date] mode. Null if there's no limit.
-  final int maximumYear;
+  final int? maximumYear;
 
   /// The granularity of the minutes spinner, if it is shown in the current mode.
   /// Must be an integer factor of 60.
@@ -355,7 +353,7 @@ class CupertinoDatePicker extends StatefulWidget {
   /// Background color of date picker.
   ///
   /// Defaults to null, which disables background painting entirely.
-  final Color backgroundColor;
+  final Color? backgroundColor;
 
   @override
   State<StatefulWidget> createState() {
@@ -370,9 +368,6 @@ class CupertinoDatePicker extends StatefulWidget {
       case CupertinoDatePickerMode.date:
         return _CupertinoDatePickerDateState();
     }
-
-    assert(false);
-    return _CupertinoDatePickerDateTimeState();
   }
 
   // Estimate the minimum width that each column needs to layout its content.
@@ -460,17 +455,17 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
   // vs iOS.
   static const double _kMaximumOffAxisFraction = 0.45;
 
-  int textDirectionFactor;
-  CupertinoLocalizations localizations;
+  late int textDirectionFactor;
+  late CupertinoLocalizations localizations;
 
   // Alignment based on text direction. The variable name is self descriptive,
   // however, when text direction is rtl, alignment is reversed.
-  Alignment alignCenterLeft;
-  Alignment alignCenterRight;
+  late Alignment alignCenterLeft;
+  late Alignment alignCenterRight;
 
   // Read this out when the state is initially created. Changes in initialDateTime
   // in the widget after first build is ignored.
-  DateTime initialDateTime;
+  late DateTime initialDateTime;
 
   // The difference in days between the initial date and the currently selected date.
   // 0 if the current mode does not involve a date.
@@ -490,7 +485,7 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
     return 0;
   }
   // The controller of the date column.
-  FixedExtentScrollController dateController;
+  late FixedExtentScrollController dateController;
 
   // The current selection of the hour picker. Values range from 0 to 23.
   int get selectedHour => _selectedHour(selectedAmPm, _selectedHourIndex);
@@ -501,7 +496,7 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
     return _isHourRegionFlipped(selectedAmPm) ? (selectedHour + 12) % 24 : selectedHour;
   }
   // The controller of the hour column.
-  FixedExtentScrollController hourController;
+  late FixedExtentScrollController hourController;
 
   // The current selection of the minute picker. Values range from 0 to 59.
   int get selectedMinute {
@@ -510,7 +505,7 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
       : initialDateTime.minute;
   }
   // The controller of the minute column.
-  FixedExtentScrollController minuteController;
+  late FixedExtentScrollController minuteController;
 
   // Whether the current meridiem selection is AM or PM.
   //
@@ -518,7 +513,7 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
   // because the meridiem picker can be scrolled **animatedly** by the hour picker
   // (e.g. if you scroll from 12 to 1 in 12h format), but the meridiem change
   // should take effect immediately, **before** the animation finishes.
-  int selectedAmPm;
+  late int selectedAmPm;
   // Whether the physical-region-to-meridiem mapping is flipped.
   bool get isHourRegionFlipped => _isHourRegionFlipped(selectedAmPm);
   bool _isHourRegionFlipped(int selectedAmPm) => selectedAmPm != meridiemRegion;
@@ -531,12 +526,12 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
   // scrolls. This variable is to keep track of the selected "physical"
   // (meridiem picker invariant) region of the hour picker. The "physical" region
   // of an item of index `i` is `i ~/ 12`.
-  int meridiemRegion;
+  late int meridiemRegion;
   // The current selection of the AM/PM picker.
   //
   // - 0 means AM
   // - 1 means PM
-  FixedExtentScrollController meridiemController;
+  late FixedExtentScrollController meridiemController;
 
   bool isDatePickerScrolling = false;
   bool isHourPickerScrolling = false;
@@ -570,7 +565,7 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
     minuteController = FixedExtentScrollController(initialItem: initialDateTime.minute ~/ widget.minuteInterval);
     dateController = FixedExtentScrollController(initialItem: 0);
 
-    PaintingBinding.instance.systemFonts.addListener(_handleSystemFontsChange);
+    PaintingBinding.instance!.systemFonts.addListener(_handleSystemFontsChange);
   }
 
   void _handleSystemFontsChange () {
@@ -589,7 +584,7 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
     minuteController.dispose();
     meridiemController.dispose();
 
-    PaintingBinding.instance.systemFonts.removeListener(_handleSystemFontsChange);
+    PaintingBinding.instance!.systemFonts.removeListener(_handleSystemFontsChange);
     super.dispose();
   }
 
@@ -615,7 +610,7 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
     super.didChangeDependencies();
 
     textDirectionFactor = Directionality.of(context) == TextDirection.ltr ? 1 : -1;
-    localizations = CupertinoLocalizations.of(context);
+    localizations = CupertinoLocalizations.of(context)!;
 
     alignCenterLeft = textDirectionFactor == 1 ? Alignment.centerLeft : Alignment.centerRight;
     alignCenterRight = textDirectionFactor == 1 ? Alignment.centerRight : Alignment.centerLeft;
@@ -630,7 +625,7 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
           CupertinoDatePicker._getColumnWidth(columnType, localizations, context);
     }
 
-    return estimatedColumnWidths[columnType.index];
+    return estimatedColumnWidths[columnType.index]!;
   }
 
   // Gets the current date time of the picker.
@@ -904,14 +899,14 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
 
     if (minCheck || maxCheck) {
       // We have minCheck === !maxCheck.
-      final DateTime targetDate = minCheck ? widget.minimumDate : widget.maximumDate;
+      final DateTime targetDate = minCheck ? widget.minimumDate! : widget.maximumDate!;
       _scrollToDate(targetDate, selectedDate);
     }
   }
 
   void _scrollToDate(DateTime newDate, DateTime fromDate) {
     assert(newDate != null);
-    SchedulerBinding.instance.addPostFrameCallback((Duration timestamp) {
+    SchedulerBinding.instance!.addPostFrameCallback((Duration timestamp) {
       if (fromDate.year != newDate.year || fromDate.month != newDate.month || fromDate.day != newDate.day) {
         _animateColumnControllerToItem(dateController, selectedDayFromInitial);
       }
@@ -997,7 +992,7 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
         id: i,
         child: pickerBuilders[i](
           offAxisFraction,
-          (BuildContext context, Widget child) {
+          (BuildContext context, Widget? child) {
             return Container(
               alignment: i == columnWidths.length - 1
                 ? alignCenterLeft
@@ -1017,7 +1012,7 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
     }
 
     return MediaQuery(
-      data: MediaQuery.of(context).copyWith(textScaleFactor: 1.0),
+      data: MediaQuery.of(context)!.copyWith(textScaleFactor: 1.0),
       child: DefaultTextStyle.merge(
         style: _kDefaultPickerTextStyle,
         child: CustomMultiChildLayout(
@@ -1033,25 +1028,25 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
 }
 
 class _CupertinoDatePickerDateState extends State<CupertinoDatePicker> {
-  int textDirectionFactor;
-  CupertinoLocalizations localizations;
+  late int textDirectionFactor;
+  late CupertinoLocalizations localizations;
 
   // Alignment based on text direction. The variable name is self descriptive,
   // however, when text direction is rtl, alignment is reversed.
-  Alignment alignCenterLeft;
-  Alignment alignCenterRight;
+  late Alignment alignCenterLeft;
+  late Alignment alignCenterRight;
 
   // The currently selected values of the picker.
-  int selectedDay;
-  int selectedMonth;
-  int selectedYear;
+  late int selectedDay;
+  late int selectedMonth;
+  late int selectedYear;
 
   // The controller of the day picker. There are cases where the selected value
   // of the picker is invalid (e.g. February 30th 2018), and this dayController
   // is responsible for jumping to a valid value.
-  FixedExtentScrollController dayController;
-  FixedExtentScrollController monthController;
-  FixedExtentScrollController yearController;
+  late FixedExtentScrollController dayController;
+  late FixedExtentScrollController monthController;
+  late FixedExtentScrollController yearController;
 
   bool isDayPickerScrolling = false;
   bool isMonthPickerScrolling = false;
@@ -1073,7 +1068,7 @@ class _CupertinoDatePickerDateState extends State<CupertinoDatePicker> {
     monthController = FixedExtentScrollController(initialItem: selectedMonth - 1);
     yearController = FixedExtentScrollController(initialItem: selectedYear);
 
-    PaintingBinding.instance.systemFonts.addListener(_handleSystemFontsChange);
+    PaintingBinding.instance!.systemFonts.addListener(_handleSystemFontsChange);
   }
 
   void _handleSystemFontsChange() {
@@ -1089,7 +1084,7 @@ class _CupertinoDatePickerDateState extends State<CupertinoDatePicker> {
     monthController.dispose();
     yearController.dispose();
 
-    PaintingBinding.instance.systemFonts.removeListener(_handleSystemFontsChange);
+    PaintingBinding.instance!.systemFonts.removeListener(_handleSystemFontsChange);
     super.dispose();
   }
 
@@ -1098,7 +1093,7 @@ class _CupertinoDatePickerDateState extends State<CupertinoDatePicker> {
     super.didChangeDependencies();
 
     textDirectionFactor = Directionality.of(context) == TextDirection.ltr ? 1 : -1;
-    localizations = CupertinoLocalizations.of(context);
+    localizations = CupertinoLocalizations.of(context)!;
 
     alignCenterLeft = textDirectionFactor == 1 ? Alignment.centerLeft : Alignment.centerRight;
     alignCenterRight = textDirectionFactor == 1 ? Alignment.centerRight : Alignment.centerLeft;
@@ -1184,8 +1179,8 @@ class _CupertinoDatePickerDateState extends State<CupertinoDatePicker> {
         },
         children: List<Widget>.generate(12, (int index) {
           final int month = index + 1;
-          final bool isInvalidMonth = (widget.minimumDate?.year == selectedYear && widget.minimumDate.month > month)
-                                   || (widget.maximumDate?.year == selectedYear && widget.maximumDate.month < month);
+          final bool isInvalidMonth = (widget.minimumDate?.year == selectedYear && widget.minimumDate!.month > month)
+                                   || (widget.maximumDate?.year == selectedYear && widget.maximumDate!.month < month);
 
           return itemPositioningBuilder(
             context,
@@ -1228,11 +1223,11 @@ class _CupertinoDatePickerDateState extends State<CupertinoDatePicker> {
           if (year < widget.minimumYear)
             return null;
 
-          if (widget.maximumYear != null && year > widget.maximumYear)
+          if (widget.maximumYear != null && year > widget.maximumYear!)
             return null;
 
-          final bool isValidYear = (widget.minimumDate == null || widget.minimumDate.year <= year)
-                                && (widget.maximumDate == null || widget.maximumDate.year >= year);
+          final bool isValidYear = (widget.minimumDate == null || widget.minimumDate!.year <= year)
+                                && (widget.maximumDate == null || widget.maximumDate!.year >= year);
 
           return itemPositioningBuilder(
             context,
@@ -1277,7 +1272,7 @@ class _CupertinoDatePickerDateState extends State<CupertinoDatePicker> {
 
     if (!minCheck || maxCheck) {
       // We have minCheck === !maxCheck.
-      final DateTime targetDate = minCheck ? widget.maximumDate : widget.minimumDate;
+      final DateTime targetDate = minCheck ? widget.maximumDate! : widget.minimumDate!;
       _scrollToDate(targetDate);
       return;
     }
@@ -1292,7 +1287,7 @@ class _CupertinoDatePickerDateState extends State<CupertinoDatePicker> {
 
   void _scrollToDate(DateTime newDate) {
     assert(newDate != null);
-    SchedulerBinding.instance.addPostFrameCallback((Duration timestamp) {
+    SchedulerBinding.instance!.addPostFrameCallback((Duration timestamp) {
       if (selectedYear != newDate.year) {
         _animateColumnControllerToItem(yearController, newDate.year);
       }
@@ -1316,30 +1311,34 @@ class _CupertinoDatePickerDateState extends State<CupertinoDatePicker> {
       case DatePickerDateOrder.mdy:
         pickerBuilders = <_ColumnBuilder>[_buildMonthPicker, _buildDayPicker, _buildYearPicker];
         columnWidths = <double>[
-          estimatedColumnWidths[_PickerColumnType.month.index],
-          estimatedColumnWidths[_PickerColumnType.dayOfMonth.index],
-          estimatedColumnWidths[_PickerColumnType.year.index]];
+          estimatedColumnWidths[_PickerColumnType.month.index]!,
+          estimatedColumnWidths[_PickerColumnType.dayOfMonth.index]!,
+          estimatedColumnWidths[_PickerColumnType.year.index]!,
+        ];
         break;
       case DatePickerDateOrder.dmy:
         pickerBuilders = <_ColumnBuilder>[_buildDayPicker, _buildMonthPicker, _buildYearPicker];
         columnWidths = <double>[
-          estimatedColumnWidths[_PickerColumnType.dayOfMonth.index],
-          estimatedColumnWidths[_PickerColumnType.month.index],
-          estimatedColumnWidths[_PickerColumnType.year.index]];
+          estimatedColumnWidths[_PickerColumnType.dayOfMonth.index]!,
+          estimatedColumnWidths[_PickerColumnType.month.index]!,
+          estimatedColumnWidths[_PickerColumnType.year.index]!,
+        ];
         break;
       case DatePickerDateOrder.ymd:
         pickerBuilders = <_ColumnBuilder>[_buildYearPicker, _buildMonthPicker, _buildDayPicker];
         columnWidths = <double>[
-          estimatedColumnWidths[_PickerColumnType.year.index],
-          estimatedColumnWidths[_PickerColumnType.month.index],
-          estimatedColumnWidths[_PickerColumnType.dayOfMonth.index]];
+          estimatedColumnWidths[_PickerColumnType.year.index]!,
+          estimatedColumnWidths[_PickerColumnType.month.index]!,
+          estimatedColumnWidths[_PickerColumnType.dayOfMonth.index]!,
+        ];
         break;
       case DatePickerDateOrder.ydm:
         pickerBuilders = <_ColumnBuilder>[_buildYearPicker, _buildDayPicker, _buildMonthPicker];
         columnWidths = <double>[
-          estimatedColumnWidths[_PickerColumnType.year.index],
-          estimatedColumnWidths[_PickerColumnType.dayOfMonth.index],
-          estimatedColumnWidths[_PickerColumnType.month.index]];
+          estimatedColumnWidths[_PickerColumnType.year.index]!,
+          estimatedColumnWidths[_PickerColumnType.dayOfMonth.index]!,
+          estimatedColumnWidths[_PickerColumnType.month.index]!,
+        ];
         break;
       default:
         assert(false, 'date order is not specified');
@@ -1358,7 +1357,7 @@ class _CupertinoDatePickerDateState extends State<CupertinoDatePicker> {
         id: i,
         child: pickerBuilders[i](
           offAxisFraction,
-          (BuildContext context, Widget child) {
+          (BuildContext context, Widget? child) {
             return Container(
               alignment: i == columnWidths.length - 1
                   ? alignCenterLeft
@@ -1376,7 +1375,7 @@ class _CupertinoDatePickerDateState extends State<CupertinoDatePicker> {
     }
 
     return MediaQuery(
-      data: MediaQuery.of(context).copyWith(textScaleFactor: 1.0),
+      data: MediaQuery.of(context)!.copyWith(textScaleFactor: 1.0),
       child: DefaultTextStyle.merge(
         style: _kDefaultPickerTextStyle,
         child: CustomMultiChildLayout(
@@ -1464,14 +1463,14 @@ class CupertinoTimerPicker extends StatefulWidget {
   /// [secondInterval] is the granularity of the second spinner. Must be a
   /// positive integer factor of 60.
   CupertinoTimerPicker({
-    Key key,
+    Key? key,
     this.mode = CupertinoTimerPickerMode.hms,
     this.initialTimerDuration = Duration.zero,
     this.minuteInterval = 1,
     this.secondInterval = 1,
     this.alignment = Alignment.center,
     this.backgroundColor,
-    @required this.onTimerDurationChanged,
+    required this.onTimerDurationChanged,
   }) : assert(mode != null),
        assert(onTimerDurationChanged != null),
        assert(initialTimerDuration >= Duration.zero),
@@ -1508,15 +1507,15 @@ class CupertinoTimerPicker extends StatefulWidget {
   /// Background color of timer picker.
   ///
   /// Defaults to null, which disables background painting entirely.
-  final Color backgroundColor;
+  final Color? backgroundColor;
 
   @override
   State<StatefulWidget> createState() => _CupertinoTimerPickerState();
 }
 
 class _CupertinoTimerPickerState extends State<CupertinoTimerPicker> {
-  TextDirection textDirection;
-  CupertinoLocalizations localizations;
+  late TextDirection textDirection;
+  late CupertinoLocalizations localizations;
   int get textDirectionFactor {
     switch (textDirection) {
       case TextDirection.ltr:
@@ -1524,25 +1523,24 @@ class _CupertinoTimerPickerState extends State<CupertinoTimerPicker> {
       case TextDirection.rtl:
         return -1;
     }
-    return 1;
   }
 
   // The currently selected values of the picker.
-  int selectedHour;
-  int selectedMinute;
-  int selectedSecond;
+  int? selectedHour;
+  late int selectedMinute;
+  int? selectedSecond;
 
   // On iOS the selected values won't be reported until the scrolling fully stops.
   // The values below are the latest selected values when the picker comes to a full stop.
-  int lastSelectedHour;
-  int lastSelectedMinute;
-  int lastSelectedSecond;
+  int? lastSelectedHour;
+  int? lastSelectedMinute;
+  int? lastSelectedSecond;
 
   final TextPainter textPainter = TextPainter();
   final List<String> numbers = List<String>.generate(10, (int i) => '${9 - i}');
-  double numberLabelWidth;
-  double numberLabelHeight;
-  double numberLabelBaseline;
+  late double numberLabelWidth;
+  late double numberLabelHeight;
+  late double numberLabelBaseline;
 
   @override
   void initState() {
@@ -1556,7 +1554,7 @@ class _CupertinoTimerPickerState extends State<CupertinoTimerPicker> {
     if (widget.mode != CupertinoTimerPickerMode.hm)
       selectedSecond = widget.initialTimerDuration.inSeconds % 60;
 
-    PaintingBinding.instance.systemFonts.addListener(_handleSystemFontsChange);
+    PaintingBinding.instance!.systemFonts.addListener(_handleSystemFontsChange);
   }
 
   void _handleSystemFontsChange() {
@@ -1569,7 +1567,7 @@ class _CupertinoTimerPickerState extends State<CupertinoTimerPicker> {
 
   @override
   void dispose() {
-    PaintingBinding.instance.systemFonts.removeListener(_handleSystemFontsChange);
+    PaintingBinding.instance!.systemFonts.removeListener(_handleSystemFontsChange);
     super.dispose();
   }
 
@@ -1587,8 +1585,8 @@ class _CupertinoTimerPickerState extends State<CupertinoTimerPicker> {
   void didChangeDependencies() {
     super.didChangeDependencies();
 
-    textDirection = Directionality.of(context);
-    localizations = CupertinoLocalizations.of(context);
+    textDirection = Directionality.of(context)!;
+    localizations = CupertinoLocalizations.of(context)!;
 
     _measureLabelMetrics();
   }
@@ -1598,7 +1596,7 @@ class _CupertinoTimerPickerState extends State<CupertinoTimerPicker> {
     final TextStyle textStyle = _textStyleFrom(context);
 
     double maxWidth = double.negativeInfinity;
-    String widestNumber;
+    String? widestNumber;
 
     // Assumes that:
     // - 2-digit numbers are always wider than 1-digit numbers.
@@ -1683,7 +1681,7 @@ class _CupertinoTimerPickerState extends State<CupertinoTimerPicker> {
 
   Widget _buildHourPicker(EdgeInsetsDirectional additionalPadding) {
     return CupertinoPicker(
-      scrollController: FixedExtentScrollController(initialItem: selectedHour),
+      scrollController: FixedExtentScrollController(initialItem: selectedHour!),
       offAxisFraction: -0.5 * textDirectionFactor,
       itemExtent: _kItemExtent,
       backgroundColor: widget.backgroundColor,
@@ -1693,7 +1691,7 @@ class _CupertinoTimerPickerState extends State<CupertinoTimerPicker> {
           selectedHour = index;
           widget.onTimerDurationChanged(
             Duration(
-              hours: selectedHour,
+              hours: selectedHour!,
               minutes: selectedMinute,
               seconds: selectedSecond ?? 0));
         });
@@ -1723,7 +1721,7 @@ class _CupertinoTimerPickerState extends State<CupertinoTimerPicker> {
           child: _buildHourPicker(additionalPadding),
         ),
         _buildLabel(
-          localizations.timerPickerHourLabel(lastSelectedHour ?? selectedHour),
+          localizations.timerPickerHourLabel(lastSelectedHour ?? selectedHour!),
           additionalPadding,
         ),
       ],
@@ -1801,7 +1799,7 @@ class _CupertinoTimerPickerState extends State<CupertinoTimerPicker> {
 
     return CupertinoPicker(
       scrollController: FixedExtentScrollController(
-        initialItem: selectedSecond ~/ widget.secondInterval,
+        initialItem: selectedSecond! ~/ widget.secondInterval,
       ),
       offAxisFraction: offAxisFraction,
       itemExtent: _kItemExtent,
@@ -1815,7 +1813,7 @@ class _CupertinoTimerPickerState extends State<CupertinoTimerPicker> {
             Duration(
               hours: selectedHour ?? 0,
               minutes: selectedMinute,
-              seconds: selectedSecond));
+              seconds: selectedSecond!));
         });
       },
       children: List<Widget>.generate(60 ~/ widget.secondInterval, (int index) {
@@ -1845,7 +1843,7 @@ class _CupertinoTimerPickerState extends State<CupertinoTimerPicker> {
           child: _buildSecondPicker(additionalPadding),
         ),
         _buildLabel(
-          localizations.timerPickerSecondLabel(lastSelectedSecond ?? selectedSecond),
+          localizations.timerPickerSecondLabel(lastSelectedSecond ?? selectedSecond!),
           additionalPadding,
         ),
       ],
@@ -1902,7 +1900,7 @@ class _CupertinoTimerPickerState extends State<CupertinoTimerPicker> {
     return MediaQuery(
       // The native iOS picker's text scaling is fixed, so we will also fix it
       // as well in our picker.
-      data: MediaQuery.of(context).copyWith(textScaleFactor: 1.0),
+      data: MediaQuery.of(context)!.copyWith(textScaleFactor: 1.0),
       child: CupertinoTheme(
         data: themeData.copyWith(
           textTheme: themeData.textTheme.copyWith(

packages/flutter/lib/src/cupertino/dialog.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 'dart:ui' show ImageFilter;
 
@@ -97,7 +95,7 @@ const double _kMaxRegularTextScaleFactor = 1.4;
 // Accessibility mode on iOS is determined by the text scale factor that the
 // user has selected.
 bool _isInAccessibilityMode(BuildContext context) {
-  final MediaQueryData data = MediaQuery.of(context, nullOk: true);
+  final MediaQueryData? data = MediaQuery.of(context, nullOk: true);
   return data != null && data.textScaleFactor > _kMaxRegularTextScaleFactor;
 }
 
@@ -131,7 +129,7 @@ class CupertinoAlertDialog extends StatelessWidget {
   ///
   /// The [actions] must not be null.
   const CupertinoAlertDialog({
-    Key key,
+    Key? key,
     this.title,
     this.content,
     this.actions = const <Widget>[],
@@ -146,13 +144,13 @@ class CupertinoAlertDialog extends StatelessWidget {
   /// of the dialog.
   ///
   /// Typically a [Text] widget.
-  final Widget title;
+  final Widget? title;
 
   /// The (optional) content of the dialog is displayed in the center of the
   /// dialog in a lighter font.
   ///
   /// Typically a [Text] widget.
-  final Widget content;
+  final Widget? content;
 
   /// The (optional) set of actions that are displayed at the bottom of the
   /// dialog.
@@ -170,7 +168,7 @@ class CupertinoAlertDialog extends StatelessWidget {
   ///
   ///  * [actionScrollController], which can be used for controlling the actions
   ///    section when there are many actions.
-  final ScrollController scrollController;
+  final ScrollController? scrollController;
 
   /// A scroll controller that can be used to control the scrolling of the
   /// actions in the dialog.
@@ -181,7 +179,7 @@ class CupertinoAlertDialog extends StatelessWidget {
   ///
   ///  * [scrollController], which can be used for controlling the [content]
   ///    section when it is long.
-  final ScrollController actionScrollController;
+  final ScrollController? actionScrollController;
 
   /// {@macro flutter.material.dialog.insetAnimationDuration}
   final Duration insetAnimationDuration;
@@ -228,20 +226,20 @@ class CupertinoAlertDialog extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final CupertinoLocalizations localizations = CupertinoLocalizations.of(context);
+    final CupertinoLocalizations localizations = CupertinoLocalizations.of(context)!;
     final bool isInAccessibilityMode = _isInAccessibilityMode(context);
-    final double textScaleFactor = MediaQuery.of(context).textScaleFactor;
+    final double textScaleFactor = MediaQuery.of(context)!.textScaleFactor;
     return CupertinoUserInterfaceLevel(
       data: CupertinoUserInterfaceLevelData.elevated,
       child: MediaQuery(
-        data: MediaQuery.of(context).copyWith(
+        data: MediaQuery.of(context)!.copyWith(
           // iOS does not shrink dialog content below a 1.0 scale factor
           textScaleFactor: math.max(textScaleFactor, 1.0),
         ),
         child: LayoutBuilder(
           builder: (BuildContext context, BoxConstraints constraints) {
             return AnimatedPadding(
-              padding: MediaQuery.of(context).viewInsets +
+              padding: MediaQuery.of(context)!.viewInsets +
                   const EdgeInsets.symmetric(horizontal: 40.0, vertical: 24.0),
               duration: insetAnimationDuration,
               curve: insetAnimationCurve,
@@ -302,12 +300,12 @@ class CupertinoAlertDialog extends StatelessWidget {
 class CupertinoDialog extends StatelessWidget {
   /// Creates an iOS-style dialog.
   const CupertinoDialog({
-    Key key,
+    Key? key,
     this.child,
   }) : super(key: key);
 
   /// The widget below this widget in the tree.
-  final Widget child;
+  final Widget? child;
 
   @override
   Widget build(BuildContext context) {
@@ -340,7 +338,7 @@ class CupertinoDialog extends StatelessWidget {
 class CupertinoPopupSurface extends StatelessWidget {
   /// Creates an iOS-style rounded rectangle popup surface.
   const CupertinoPopupSurface({
-    Key key,
+    Key? key,
     this.isSurfacePainted = true,
     this.child,
   }) : super(key: key);
@@ -356,7 +354,7 @@ class CupertinoPopupSurface extends StatelessWidget {
   final bool isSurfacePainted;
 
   /// The widget below this widget in the tree.
-  final Widget child;
+  final Widget? child;
 
   @override
   Widget build(BuildContext context) {
@@ -379,9 +377,9 @@ class CupertinoPopupSurface extends StatelessWidget {
 // See [_RenderCupertinoDialog] for specific layout policy details.
 class _CupertinoDialogRenderWidget extends RenderObjectWidget {
   const _CupertinoDialogRenderWidget({
-    Key key,
-    @required this.contentSection,
-    @required this.actionsSection,
+    Key? key,
+    required this.contentSection,
+    required this.actionsSection,
   }) : super(key: key);
 
   final Widget contentSection;
@@ -390,9 +388,9 @@ class _CupertinoDialogRenderWidget extends RenderObjectWidget {
   @override
   RenderObject createRenderObject(BuildContext context) {
     return _RenderCupertinoDialog(
-      dividerThickness: _kDividerThickness / MediaQuery.of(context).devicePixelRatio,
+      dividerThickness: _kDividerThickness / MediaQuery.of(context)!.devicePixelRatio,
       isInAccessibilityMode: _isInAccessibilityMode(context),
-      dividerColor: CupertinoDynamicColor.resolve(CupertinoColors.separator, context),
+      dividerColor: CupertinoDynamicColor.resolve(CupertinoColors.separator, context)!,
     );
   }
 
@@ -400,7 +398,7 @@ class _CupertinoDialogRenderWidget extends RenderObjectWidget {
   void updateRenderObject(BuildContext context, _RenderCupertinoDialog renderObject) {
     renderObject
       ..isInAccessibilityMode = _isInAccessibilityMode(context)
-      ..dividerColor = CupertinoDynamicColor.resolve(CupertinoColors.separator, context);
+      ..dividerColor = CupertinoDynamicColor.resolve(CupertinoColors.separator, context)!;
   }
 
   @override
@@ -412,8 +410,8 @@ class _CupertinoDialogRenderWidget extends RenderObjectWidget {
 class _CupertinoDialogRenderElement extends RenderObjectElement {
   _CupertinoDialogRenderElement(_CupertinoDialogRenderWidget widget) : super(widget);
 
-  Element _contentElement;
-  Element _actionsElement;
+  Element? _contentElement;
+  Element? _actionsElement;
 
   @override
   _CupertinoDialogRenderWidget get widget => super.widget as _CupertinoDialogRenderWidget;
@@ -424,15 +422,15 @@ class _CupertinoDialogRenderElement extends RenderObjectElement {
   @override
   void visitChildren(ElementVisitor visitor) {
     if (_contentElement != null) {
-      visitor(_contentElement);
+      visitor(_contentElement!);
     }
     if (_actionsElement != null) {
-      visitor(_actionsElement);
+      visitor(_actionsElement!);
     }
   }
 
   @override
-  void mount(Element parent, dynamic newSlot) {
+  void mount(Element? parent, dynamic newSlot) {
     super.mount(parent, newSlot);
     _contentElement = updateChild(_contentElement, widget.contentSection, _AlertDialogSections.contentSection);
     _actionsElement = updateChild(_actionsElement, widget.actionsSection, _AlertDialogSections.actionsSection);
@@ -513,11 +511,11 @@ class _CupertinoDialogRenderElement extends RenderObjectElement {
 // section is given whatever height remains.
 class _RenderCupertinoDialog extends RenderBox {
   _RenderCupertinoDialog({
-    RenderBox contentSection,
-    RenderBox actionsSection,
+    RenderBox? contentSection,
+    RenderBox? actionsSection,
     double dividerThickness = 0.0,
     bool isInAccessibilityMode = false,
-    Color dividerColor,
+    required Color dividerColor,
   }) : _contentSection = contentSection,
        _actionsSection = actionsSection,
        _dividerThickness = dividerThickness,
@@ -527,30 +525,30 @@ class _RenderCupertinoDialog extends RenderBox {
         ..style = PaintingStyle.fill;
 
 
-  RenderBox get contentSection => _contentSection;
-  RenderBox _contentSection;
-  set contentSection(RenderBox newContentSection) {
+  RenderBox? get contentSection => _contentSection;
+  RenderBox? _contentSection;
+  set contentSection(RenderBox? newContentSection) {
     if (newContentSection != _contentSection) {
       if (_contentSection != null) {
-        dropChild(_contentSection);
+        dropChild(_contentSection!);
       }
       _contentSection = newContentSection;
       if (_contentSection != null) {
-        adoptChild(_contentSection);
+        adoptChild(_contentSection!);
       }
     }
   }
 
-  RenderBox get actionsSection => _actionsSection;
-  RenderBox _actionsSection;
-  set actionsSection(RenderBox newActionsSection) {
+  RenderBox? get actionsSection => _actionsSection;
+  RenderBox? _actionsSection;
+  set actionsSection(RenderBox? newActionsSection) {
     if (newActionsSection != _actionsSection) {
       if (null != _actionsSection) {
-        dropChild(_actionsSection);
+        dropChild(_actionsSection!);
       }
       _actionsSection = newActionsSection;
       if (null != _actionsSection) {
-        adoptChild(_actionsSection);
+        adoptChild(_actionsSection!);
       }
     }
   }
@@ -585,10 +583,10 @@ class _RenderCupertinoDialog extends RenderBox {
   void attach(PipelineOwner owner) {
     super.attach(owner);
     if (null != contentSection) {
-      contentSection.attach(owner);
+      contentSection!.attach(owner);
     }
     if (null != actionsSection) {
-      actionsSection.attach(owner);
+      actionsSection!.attach(owner);
     }
   }
 
@@ -596,20 +594,20 @@ class _RenderCupertinoDialog extends RenderBox {
   void detach() {
     super.detach();
     if (null != contentSection) {
-      contentSection.detach();
+      contentSection!.detach();
     }
     if (null != actionsSection) {
-      actionsSection.detach();
+      actionsSection!.detach();
     }
   }
 
   @override
   void redepthChildren() {
     if (null != contentSection) {
-      redepthChild(contentSection);
+      redepthChild(contentSection!);
     }
     if (null != actionsSection) {
-      redepthChild(actionsSection);
+      redepthChild(actionsSection!);
     }
   }
 
@@ -623,17 +621,17 @@ class _RenderCupertinoDialog extends RenderBox {
   @override
   void visitChildren(RenderObjectVisitor visitor) {
     if (contentSection != null) {
-      visitor(contentSection);
+      visitor(contentSection!);
     }
     if (actionsSection != null) {
-      visitor(actionsSection);
+      visitor(actionsSection!);
     }
   }
 
   @override
   List<DiagnosticsNode> debugDescribeChildren() => <DiagnosticsNode>[
-    if (contentSection != null) contentSection.toDiagnosticsNode(name: 'content'),
-    if (actionsSection != null) actionsSection.toDiagnosticsNode(name: 'actions'),
+    if (contentSection != null) contentSection!.toDiagnosticsNode(name: 'content'),
+    if (actionsSection != null) actionsSection!.toDiagnosticsNode(name: 'actions'),
   ];
 
   @override
@@ -648,8 +646,8 @@ class _RenderCupertinoDialog extends RenderBox {
 
   @override
   double computeMinIntrinsicHeight(double width) {
-    final double contentHeight = contentSection.getMinIntrinsicHeight(width);
-    final double actionsHeight = actionsSection.getMinIntrinsicHeight(width);
+    final double contentHeight = contentSection!.getMinIntrinsicHeight(width);
+    final double actionsHeight = actionsSection!.getMinIntrinsicHeight(width);
     final bool hasDivider = contentHeight > 0.0 && actionsHeight > 0.0;
     final double height = contentHeight + (hasDivider ? _dividerThickness : 0.0) + actionsHeight;
 
@@ -660,8 +658,8 @@ class _RenderCupertinoDialog extends RenderBox {
 
   @override
   double computeMaxIntrinsicHeight(double width) {
-    final double contentHeight = contentSection.getMaxIntrinsicHeight(width);
-    final double actionsHeight = actionsSection.getMaxIntrinsicHeight(width);
+    final double contentHeight = contentSection!.getMaxIntrinsicHeight(width);
+    final double actionsHeight = actionsSection!.getMaxIntrinsicHeight(width);
     final bool hasDivider = contentHeight > 0.0 && actionsHeight > 0.0;
     final double height = contentHeight + (hasDivider ? _dividerThickness : 0.0) + actionsHeight;
 
@@ -685,25 +683,25 @@ class _RenderCupertinoDialog extends RenderBox {
   }
 
   void performRegularLayout() {
-    final bool hasDivider = contentSection.getMaxIntrinsicHeight(_dialogWidth) > 0.0
-        && actionsSection.getMaxIntrinsicHeight(_dialogWidth) > 0.0;
+    final bool hasDivider = contentSection!.getMaxIntrinsicHeight(_dialogWidth) > 0.0
+        && actionsSection!.getMaxIntrinsicHeight(_dialogWidth) > 0.0;
     final double dividerThickness = hasDivider ? _dividerThickness : 0.0;
 
-    final double minActionsHeight = actionsSection.getMinIntrinsicHeight(_dialogWidth);
+    final double minActionsHeight = actionsSection!.getMinIntrinsicHeight(_dialogWidth);
 
     // Size alert dialog content.
-    contentSection.layout(
+    contentSection!.layout(
       constraints.deflate(EdgeInsets.only(bottom: minActionsHeight + dividerThickness)),
       parentUsesSize: true,
     );
-    final Size contentSize = contentSection.size;
+    final Size contentSize = contentSection!.size;
 
     // Size alert dialog actions.
-    actionsSection.layout(
+    actionsSection!.layout(
       constraints.deflate(EdgeInsets.only(top: contentSize.height + dividerThickness)),
       parentUsesSize: true,
     );
-    final Size actionsSize = actionsSection.size;
+    final Size actionsSize = actionsSection!.size;
 
     // Calculate overall dialog height.
     final double dialogHeight = contentSize.height + dividerThickness + actionsSize.height;
@@ -715,18 +713,18 @@ class _RenderCupertinoDialog extends RenderBox {
 
     // Set the position of the actions box to sit at the bottom of the dialog.
     // The content box defaults to the top left, which is where we want it.
-    assert(actionsSection.parentData is BoxParentData);
-    final BoxParentData actionParentData = actionsSection.parentData as BoxParentData;
+    assert(actionsSection!.parentData is BoxParentData);
+    final BoxParentData actionParentData = actionsSection!.parentData as BoxParentData;
     actionParentData.offset = Offset(0.0, contentSize.height + dividerThickness);
   }
 
   void performAccessibilityLayout() {
-    final bool hasDivider = contentSection.getMaxIntrinsicHeight(_dialogWidth) > 0.0
-        && actionsSection.getMaxIntrinsicHeight(_dialogWidth) > 0.0;
+    final bool hasDivider = contentSection!.getMaxIntrinsicHeight(_dialogWidth) > 0.0
+        && actionsSection!.getMaxIntrinsicHeight(_dialogWidth) > 0.0;
     final double dividerThickness = hasDivider ? _dividerThickness : 0.0;
 
-    final double maxContentHeight = contentSection.getMaxIntrinsicHeight(_dialogWidth);
-    final double maxActionsHeight = actionsSection.getMaxIntrinsicHeight(_dialogWidth);
+    final double maxContentHeight = contentSection!.getMaxIntrinsicHeight(_dialogWidth);
+    final double maxActionsHeight = actionsSection!.getMaxIntrinsicHeight(_dialogWidth);
 
     Size contentSize;
     Size actionsSize;
@@ -737,34 +735,34 @@ class _RenderCupertinoDialog extends RenderBox {
       // section.
 
       // Size alert dialog actions.
-      actionsSection.layout(
+      actionsSection!.layout(
         constraints.deflate(EdgeInsets.only(top: constraints.maxHeight / 2.0)),
         parentUsesSize: true,
       );
-      actionsSize = actionsSection.size;
+      actionsSize = actionsSection!.size;
 
       // Size alert dialog content.
-      contentSection.layout(
+      contentSection!.layout(
         constraints.deflate(EdgeInsets.only(bottom: actionsSize.height + dividerThickness)),
         parentUsesSize: true,
       );
-      contentSize = contentSection.size;
+      contentSize = contentSection!.size;
     } else {
       // Everything fits. Give content and actions all the space they want.
 
       // Size alert dialog content.
-      contentSection.layout(
+      contentSection!.layout(
         constraints,
         parentUsesSize: true,
       );
-      contentSize = contentSection.size;
+      contentSize = contentSection!.size;
 
       // Size alert dialog actions.
-      actionsSection.layout(
+      actionsSection!.layout(
         constraints.deflate(EdgeInsets.only(top: contentSize.height)),
         parentUsesSize: true,
       );
-      actionsSize = actionsSection.size;
+      actionsSize = actionsSection!.size;
     }
 
     // Calculate overall dialog height.
@@ -777,30 +775,30 @@ class _RenderCupertinoDialog extends RenderBox {
 
     // Set the position of the actions box to sit at the bottom of the dialog.
     // The content box defaults to the top left, which is where we want it.
-    assert(actionsSection.parentData is BoxParentData);
-    final BoxParentData actionParentData = actionsSection.parentData as BoxParentData;
+    assert(actionsSection!.parentData is BoxParentData);
+    final BoxParentData actionParentData = actionsSection!.parentData as BoxParentData;
     actionParentData.offset = Offset(0.0, contentSize.height + dividerThickness);
   }
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    final BoxParentData contentParentData = contentSection.parentData as BoxParentData;
-    contentSection.paint(context, offset + contentParentData.offset);
+    final BoxParentData contentParentData = contentSection!.parentData as BoxParentData;
+    contentSection!.paint(context, offset + contentParentData.offset);
 
-    final bool hasDivider = contentSection.size.height > 0.0 && actionsSection.size.height > 0.0;
+    final bool hasDivider = contentSection!.size.height > 0.0 && actionsSection!.size.height > 0.0;
     if (hasDivider) {
       _paintDividerBetweenContentAndActions(context.canvas, offset);
     }
 
-    final BoxParentData actionsParentData = actionsSection.parentData as BoxParentData;
-    actionsSection.paint(context, offset + actionsParentData.offset);
+    final BoxParentData actionsParentData = actionsSection!.parentData as BoxParentData;
+    actionsSection!.paint(context, offset + actionsParentData.offset);
   }
 
   void _paintDividerBetweenContentAndActions(Canvas canvas, Offset offset) {
     canvas.drawRect(
       Rect.fromLTWH(
         offset.dx,
-        offset.dy + contentSection.size.height,
+        offset.dy + contentSection!.size.height,
         size.width,
         _dividerThickness,
       ),
@@ -809,15 +807,15 @@ class _RenderCupertinoDialog extends RenderBox {
   }
 
   @override
-  bool hitTestChildren(BoxHitTestResult result, { Offset position }) {
-    final BoxParentData contentSectionParentData = contentSection.parentData as BoxParentData;
-    final BoxParentData actionsSectionParentData = actionsSection.parentData as BoxParentData;
+  bool hitTestChildren(BoxHitTestResult result, { required Offset position }) {
+    final BoxParentData contentSectionParentData = contentSection!.parentData as BoxParentData;
+    final BoxParentData actionsSectionParentData = actionsSection!.parentData as BoxParentData;
     return result.addWithPaintOffset(
              offset: contentSectionParentData.offset,
              position: position,
              hitTest: (BoxHitTestResult result, Offset transformed) {
                assert(transformed == position - contentSectionParentData.offset);
-               return contentSection.hitTest(result, position: transformed);
+               return contentSection!.hitTest(result, position: transformed);
              },
            )
         || result.addWithPaintOffset(
@@ -825,7 +823,7 @@ class _RenderCupertinoDialog extends RenderBox {
              position: position,
              hitTest: (BoxHitTestResult result, Offset transformed) {
                assert(transformed == position - actionsSectionParentData.offset);
-               return actionsSection.hitTest(result, position: transformed);
+               return actionsSection!.hitTest(result, position: transformed);
              },
            );
   }
@@ -845,7 +843,7 @@ enum _AlertDialogSections {
 // a SingleChildScrollView with a zero-sized Container.
 class _CupertinoAlertContentSection extends StatelessWidget {
   const _CupertinoAlertContentSection({
-    Key key,
+    Key? key,
     this.title,
     this.content,
     this.scrollController,
@@ -855,20 +853,20 @@ class _CupertinoAlertContentSection extends StatelessWidget {
   // of the dialog.
   //
   // Typically a Text widget.
-  final Widget title;
+  final Widget? title;
 
   // The (optional) content of the dialog is displayed in the center of the
   // dialog in a lighter font.
   //
   // Typically a Text widget.
-  final Widget content;
+  final Widget? content;
 
   // A scroll controller that can be used to control the scrolling of the
   // content in the dialog.
   //
   // Defaults to null, and is typically not needed, since most alert contents
   // are short.
-  final ScrollController scrollController;
+  final ScrollController? scrollController;
 
   @override
   Widget build(BuildContext context) {
@@ -879,7 +877,7 @@ class _CupertinoAlertContentSection extends StatelessWidget {
       );
     }
 
-    final double textScaleFactor = MediaQuery.of(context).textScaleFactor;
+    final double textScaleFactor = MediaQuery.of(context)!.textScaleFactor;
     final List<Widget> titleContentGroup = <Widget>[
       if (title != null)
         Padding(
@@ -894,7 +892,7 @@ class _CupertinoAlertContentSection extends StatelessWidget {
               color: CupertinoDynamicColor.resolve(CupertinoColors.label, context),
             ),
             textAlign: TextAlign.center,
-            child: title,
+            child: title!,
           ),
         ),
       if (content != null)
@@ -910,7 +908,7 @@ class _CupertinoAlertContentSection extends StatelessWidget {
               color: CupertinoDynamicColor.resolve(CupertinoColors.label, context),
             ),
             textAlign: TextAlign.center,
-            child: content,
+            child: content!,
           ),
         ),
     ];
@@ -934,8 +932,8 @@ class _CupertinoAlertContentSection extends StatelessWidget {
 // and layout.
 class _CupertinoAlertActionSection extends StatefulWidget {
   const _CupertinoAlertActionSection({
-    Key key,
-    @required this.children,
+    Key? key,
+    required this.children,
     this.scrollController,
   }) : assert(children != null),
        super(key: key);
@@ -947,7 +945,7 @@ class _CupertinoAlertActionSection extends StatefulWidget {
   //
   // Defaults to null, and is typically not needed, since most alert dialogs
   // don't have many actions.
-  final ScrollController scrollController;
+  final ScrollController? scrollController;
 
   @override
   _CupertinoAlertActionSectionState createState() => _CupertinoAlertActionSectionState();
@@ -956,7 +954,7 @@ class _CupertinoAlertActionSection extends StatefulWidget {
 class _CupertinoAlertActionSectionState extends State<_CupertinoAlertActionSection> {
   @override
   Widget build(BuildContext context) {
-    final double devicePixelRatio = MediaQuery.of(context).devicePixelRatio;
+    final double devicePixelRatio = MediaQuery.of(context)!.devicePixelRatio;
 
     final List<Widget> interactiveButtons = <Widget>[];
     for (int i = 0; i < widget.children.length; i += 1) {
@@ -986,7 +984,7 @@ class _CupertinoAlertActionSectionState extends State<_CupertinoAlertActionSecti
 // appropriately by _RenderCupertinoDialogActions.
 class _PressableActionButton extends StatefulWidget {
   const _PressableActionButton({
-    @required this.child,
+    required this.child,
   });
 
   final Widget child;
@@ -1033,9 +1031,9 @@ class _PressableActionButtonState extends State<_PressableActionButton> {
 // incoming [isPressed] property.
 class _ActionButtonParentDataWidget extends ParentDataWidget<_ActionButtonParentData> {
   const _ActionButtonParentDataWidget({
-    Key key,
-    this.isPressed,
-    @required Widget child,
+    Key? key,
+    required this.isPressed,
+    required Widget child,
   }) : super(key: key, child: child);
 
   final bool isPressed;
@@ -1048,7 +1046,7 @@ class _ActionButtonParentDataWidget extends ParentDataWidget<_ActionButtonParent
       parentData.isPressed = isPressed;
 
       // Force a repaint.
-      final AbstractNode targetParent = renderObject.parent;
+      final AbstractNode? targetParent = renderObject.parent;
       if (targetParent is RenderObject)
         targetParent.markNeedsPaint();
     }
@@ -1077,12 +1075,12 @@ class _ActionButtonParentData extends MultiChildLayoutParentData {
 class CupertinoDialogAction extends StatelessWidget {
   /// Creates an action for an iOS-style dialog.
   const CupertinoDialogAction({
-    Key key,
+    Key? key,
     this.onPressed,
     this.isDefaultAction = false,
     this.isDestructiveAction = false,
     this.textStyle,
-    @required this.child,
+    required this.child,
   }) : assert(child != null),
        assert(isDefaultAction != null),
        assert(isDestructiveAction != null),
@@ -1092,7 +1090,7 @@ class CupertinoDialogAction extends StatelessWidget {
   /// activated.
   ///
   /// If this is set to null, the button will be disabled.
-  final VoidCallback onPressed;
+  final VoidCallback? onPressed;
 
   /// Set to true if button is the default choice in the dialog.
   ///
@@ -1117,7 +1115,7 @@ class CupertinoDialogAction extends StatelessWidget {
   /// ensure that this resizing policy always works as expected, [textStyle]
   /// must be used if a text size is desired other than that specified in
   /// [_kCupertinoDialogActionStyle].
-  final TextStyle textStyle;
+  final TextStyle? textStyle;
 
   /// The widget below this widget in the tree.
   ///
@@ -1138,9 +1136,9 @@ class CupertinoDialogAction extends StatelessWidget {
   //
   // This policy only applies when the device is not in accessibility mode.
   Widget _buildContentWithRegularSizingPolicy({
-    @required BuildContext context,
-    @required TextStyle textStyle,
-    @required Widget content,
+    required BuildContext context,
+    required TextStyle textStyle,
+    required Widget content,
   }) {
     final bool isInAccessibilityMode = _isInAccessibilityMode(context);
     final double dialogWidth = isInAccessibilityMode
@@ -1151,7 +1149,7 @@ class CupertinoDialogAction extends StatelessWidget {
     // iOS scale factor) vs the minimum text size that we allow in action
     // buttons. This ratio information is used to automatically scale down action
     // button text to fit the available space.
-    final double fontSizeRatio = (textScaleFactor * textStyle.fontSize) / _kMinButtonFontSize;
+    final double fontSizeRatio = (textScaleFactor * textStyle.fontSize!) / _kMinButtonFontSize;
     final double padding = _calculatePadding(context);
 
     return IntrinsicHeight(
@@ -1184,8 +1182,8 @@ class CupertinoDialogAction extends StatelessWidget {
   // accessibility mode. If text is used as the content, the text wraps instead
   // of ellipsizing.
   Widget _buildContentWithAccessibilitySizingPolicy({
-    @required TextStyle textStyle,
-    @required Widget content,
+    required TextStyle textStyle,
+    required Widget content,
   }) {
     return DefaultTextStyle(
       style: textStyle,
@@ -1209,7 +1207,7 @@ class CupertinoDialogAction extends StatelessWidget {
     }
 
     if (!enabled) {
-      style = style.copyWith(color: style.color.withOpacity(0.5));
+      style = style.copyWith(color: style.color!.withOpacity(0.5));
     }
 
     // Apply a sizing policy to the action button's content based on whether or
@@ -1256,8 +1254,8 @@ class CupertinoDialogAction extends StatelessWidget {
 // See [_RenderCupertinoDialogActions] for specific layout policy details.
 class _CupertinoDialogActionsRenderWidget extends MultiChildRenderObjectWidget {
   _CupertinoDialogActionsRenderWidget({
-    Key key,
-    @required List<Widget> actionButtons,
+    Key? key,
+    required List<Widget> actionButtons,
     double dividerThickness = 0.0,
   }) : _dividerThickness = dividerThickness,
        super(key: key, children: actionButtons);
@@ -1271,9 +1269,9 @@ class _CupertinoDialogActionsRenderWidget extends MultiChildRenderObjectWidget {
         ? _kAccessibilityCupertinoDialogWidth
         : _kCupertinoDialogWidth,
       dividerThickness: _dividerThickness,
-      dialogColor: CupertinoDynamicColor.resolve(_kDialogColor, context),
-      dialogPressedColor: CupertinoDynamicColor.resolve(_kDialogPressedColor, context),
-      dividerColor: CupertinoDynamicColor.resolve(CupertinoColors.separator, context),
+      dialogColor: CupertinoDynamicColor.resolve(_kDialogColor, context)!,
+      dialogPressedColor: CupertinoDynamicColor.resolve(_kDialogPressedColor, context)!,
+      dividerColor: CupertinoDynamicColor.resolve(CupertinoColors.separator, context)!,
     );
   }
 
@@ -1284,9 +1282,9 @@ class _CupertinoDialogActionsRenderWidget extends MultiChildRenderObjectWidget {
         ? _kAccessibilityCupertinoDialogWidth
         : _kCupertinoDialogWidth
       ..dividerThickness = _dividerThickness
-      ..dialogColor = CupertinoDynamicColor.resolve(_kDialogColor, context)
-      ..dialogPressedColor = CupertinoDynamicColor.resolve(_kDialogPressedColor, context)
-      ..dividerColor = CupertinoDynamicColor.resolve(CupertinoColors.separator, context);
+      ..dialogColor = CupertinoDynamicColor.resolve(_kDialogColor, context)!
+      ..dialogPressedColor = CupertinoDynamicColor.resolve(_kDialogPressedColor, context)!
+      ..dividerColor = CupertinoDynamicColor.resolve(CupertinoColors.separator, context)!;
   }
 }
 
@@ -1328,12 +1326,12 @@ class _RenderCupertinoDialogActions extends RenderBox
     with ContainerRenderObjectMixin<RenderBox, MultiChildLayoutParentData>,
         RenderBoxContainerDefaultsMixin<RenderBox, MultiChildLayoutParentData> {
   _RenderCupertinoDialogActions({
-    List<RenderBox> children,
-    @required double dialogWidth,
+    List<RenderBox>? children,
+    required double dialogWidth,
     double dividerThickness = 0.0,
-    @required Color dialogColor,
-    @required Color dialogPressedColor,
-    @required Color dividerColor,
+    required Color dialogColor,
+    required Color dialogPressedColor,
+    required Color dividerColor,
   }) : _dialogWidth = dialogWidth,
        _buttonBackgroundPaint = Paint()
         ..color = dialogColor
@@ -1395,7 +1393,7 @@ class _RenderCupertinoDialogActions extends RenderBox
   }
 
   Iterable<RenderBox> get _pressedButtons sync* {
-    RenderBox currentChild = firstChild;
+    RenderBox? currentChild = firstChild;
     while (currentChild != null) {
       assert(currentChild.parentData is _ActionButtonParentData);
       final _ActionButtonParentData parentData = currentChild.parentData as _ActionButtonParentData;
@@ -1407,7 +1405,7 @@ class _RenderCupertinoDialogActions extends RenderBox
   }
 
   bool get _isButtonPressed {
-    RenderBox currentChild = firstChild;
+    RenderBox? currentChild = firstChild;
     while (currentChild != null) {
       assert(currentChild.parentData is _ActionButtonParentData);
       final _ActionButtonParentData parentData = currentChild.parentData as _ActionButtonParentData;
@@ -1463,12 +1461,12 @@ class _RenderCupertinoDialogActions extends RenderBox
 
     double minHeight;
     if (childCount == 1) {
-      minHeight = firstChild.getMinIntrinsicHeight(width);
+      minHeight = firstChild!.getMinIntrinsicHeight(width);
     } else {
       final double perButtonWidth = (width - dividerThickness) / 2.0;
       minHeight = math.max(
-        firstChild.getMinIntrinsicHeight(perButtonWidth),
-        lastChild.getMinIntrinsicHeight(perButtonWidth),
+        firstChild!.getMinIntrinsicHeight(perButtonWidth),
+        lastChild!.getMinIntrinsicHeight(perButtonWidth),
       );
     }
     return minHeight;
@@ -1479,9 +1477,9 @@ class _RenderCupertinoDialogActions extends RenderBox
   double _computeMinIntrinsicHeightStacked(double width) {
     assert(childCount >= 2);
 
-    return firstChild.getMinIntrinsicHeight(width)
+    return firstChild!.getMinIntrinsicHeight(width)
       + dividerThickness
-      + (0.5 * childAfter(firstChild).getMinIntrinsicHeight(width));
+      + (0.5 * childAfter(firstChild!)!.getMinIntrinsicHeight(width));
   }
 
   @override
@@ -1492,7 +1490,7 @@ class _RenderCupertinoDialogActions extends RenderBox
       maxHeight = 0.0;
     } else if (childCount == 1) {
       // One button. Our max intrinsic height is equal to the button's.
-      maxHeight = firstChild.getMaxIntrinsicHeight(width);
+      maxHeight = firstChild!.getMaxIntrinsicHeight(width);
     } else if (childCount == 2) {
       // Two buttons...
       if (_isSingleButtonRow(width)) {
@@ -1500,8 +1498,8 @@ class _RenderCupertinoDialogActions extends RenderBox
         // to the taller of the 2 buttons.
         final double perButtonWidth = (width - dividerThickness) / 2.0;
         maxHeight = math.max(
-          firstChild.getMaxIntrinsicHeight(perButtonWidth),
-          lastChild.getMaxIntrinsicHeight(perButtonWidth),
+          firstChild!.getMaxIntrinsicHeight(perButtonWidth),
+          lastChild!.getMaxIntrinsicHeight(perButtonWidth),
         );
       } else {
         // The 2 buttons do not fit side by side. Measure total height as a
@@ -1523,7 +1521,7 @@ class _RenderCupertinoDialogActions extends RenderBox
 
     final double allDividersHeight = (childCount - 1) * dividerThickness;
     double heightAccumulation = allDividersHeight;
-    RenderBox button = firstChild;
+    RenderBox? button = firstChild;
     while (button != null) {
       heightAccumulation += button.getMaxIntrinsicHeight(width);
       button = childAfter(button);
@@ -1538,9 +1536,9 @@ class _RenderCupertinoDialogActions extends RenderBox
     } else if (childCount == 2) {
       // There are 2 buttons. If they can fit side-by-side then that's what
       // we want to do. Otherwise, stack them vertically.
-      final double sideBySideWidth = firstChild.getMaxIntrinsicWidth(double.infinity)
+      final double sideBySideWidth = firstChild!.getMaxIntrinsicWidth(double.infinity)
           + dividerThickness
-          + lastChild.getMaxIntrinsicWidth(double.infinity);
+          + lastChild!.getMaxIntrinsicWidth(double.infinity);
       isSingleButtonRow = sideBySideWidth <= width;
     } else {
       isSingleButtonRow = false;
@@ -1555,13 +1553,13 @@ class _RenderCupertinoDialogActions extends RenderBox
       if (childCount == 1) {
         // We have 1 button. Our size is the width of the dialog and the height
         // of the single button.
-        firstChild.layout(
+        firstChild!.layout(
           constraints,
           parentUsesSize: true,
         );
 
         size = constraints.constrain(
-          Size(dialogWidth, firstChild.size.height)
+          Size(dialogWidth, firstChild!.size.height)
         );
       } else {
         // Each button gets half the available width, minus a single divider.
@@ -1573,27 +1571,27 @@ class _RenderCupertinoDialogActions extends RenderBox
         );
 
         // Layout the 2 buttons.
-        firstChild.layout(
+        firstChild!.layout(
           perButtonConstraints,
           parentUsesSize: true,
         );
-        lastChild.layout(
+        lastChild!.layout(
           perButtonConstraints,
           parentUsesSize: true,
         );
 
         // The 2nd button needs to be offset to the right.
-        assert(lastChild.parentData is MultiChildLayoutParentData);
-        final MultiChildLayoutParentData secondButtonParentData = lastChild.parentData as MultiChildLayoutParentData;
-        secondButtonParentData.offset = Offset(firstChild.size.width + dividerThickness, 0.0);
+        assert(lastChild!.parentData is MultiChildLayoutParentData);
+        final MultiChildLayoutParentData secondButtonParentData = lastChild!.parentData as MultiChildLayoutParentData;
+        secondButtonParentData.offset = Offset(firstChild!.size.width + dividerThickness, 0.0);
 
         // Calculate our size based on the button sizes.
         size = constraints.constrain(
           Size(
             dialogWidth,
             math.max(
-              firstChild.size.height,
-              lastChild.size.height,
+              firstChild!.size.height,
+              lastChild!.size.height,
             ),
           ),
         );
@@ -1605,7 +1603,7 @@ class _RenderCupertinoDialogActions extends RenderBox
         maxHeight: double.infinity,
       );
 
-      RenderBox child = firstChild;
+      RenderBox? child = firstChild;
       int index = 0;
       double verticalOffset = 0.0;
       while (child != null) {
@@ -1654,12 +1652,12 @@ class _RenderCupertinoDialogActions extends RenderBox
     // left or right button is pressed.
     final Rect verticalDivider = childCount == 2 && !_isButtonPressed
       ? Rect.fromLTWH(
-          offset.dx + firstChild.size.width,
+          offset.dx + firstChild!.size.width,
           offset.dy,
           dividerThickness,
           math.max(
-            firstChild.size.height,
-            lastChild.size.height,
+            firstChild!.size.height,
+            lastChild!.size.height,
           ),
         )
       : Rect.zero;
@@ -1724,8 +1722,8 @@ class _RenderCupertinoDialogActions extends RenderBox
 
     Offset accumulatingOffset = offset;
 
-    RenderBox child = firstChild;
-    RenderBox prevChild;
+    RenderBox? child = firstChild;
+    RenderBox? prevChild;
     while (child != null) {
       assert(child.parentData is _ActionButtonParentData);
       final _ActionButtonParentData currentButtonParentData = child.parentData as _ActionButtonParentData;
@@ -1782,7 +1780,7 @@ class _RenderCupertinoDialogActions extends RenderBox
   }
 
   void _drawButtons(PaintingContext context, Offset offset) {
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     while (child != null) {
       final MultiChildLayoutParentData childParentData = child.parentData as MultiChildLayoutParentData;
       context.paintChild(child, childParentData.offset + offset);
@@ -1791,7 +1789,7 @@ class _RenderCupertinoDialogActions extends RenderBox
   }
 
   @override
-  bool hitTestChildren(BoxHitTestResult result, { Offset position }) {
+  bool hitTestChildren(BoxHitTestResult result, { required Offset position }) {
     return defaultHitTestChildren(result, position: position);
   }
 }

packages/flutter/lib/src/cupertino/localizations.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';
 
@@ -156,7 +154,7 @@ abstract class CupertinoLocalizations {
   /// there are, e.g. 'tab, 1 of 2' in United States English.
   ///
   /// `tabIndex` and `tabCount` must be greater than or equal to one.
-  String tabSemanticsLabel({int tabIndex, int tabCount});
+  String tabSemanticsLabel({required int tabIndex, required int tabCount});
 
   /// Hour that is shown in [CupertinoTimerPicker] corresponding to
   /// the given hour value.
@@ -241,7 +239,7 @@ abstract class CupertinoLocalizations {
   /// ```dart
   /// CupertinoLocalizations.of(context).anteMeridiemAbbreviation;
   /// ```
-  static CupertinoLocalizations of(BuildContext context) {
+  static CupertinoLocalizations? of(BuildContext context) {
     return Localizations.of<CupertinoLocalizations>(context, CupertinoLocalizations);
   }
 }
@@ -364,7 +362,7 @@ class DefaultCupertinoLocalizations implements CupertinoLocalizations {
   String get alertDialogLabel => 'Alert';
 
   @override
-  String tabSemanticsLabel({int tabIndex, int tabCount}) {
+  String tabSemanticsLabel({required int tabIndex, required int tabCount}) {
     assert(tabIndex >= 1);
     assert(tabCount >= 1);
     return 'Tab $tabIndex of $tabCount';

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

@@ -2,16 +2,14 @@
 // 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';
 import 'dart:ui' show lerpDouble, ImageFilter;
 
+import 'package:flutter/animation.dart' show Curves;
 import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
-import 'package:flutter/animation.dart' show Curves;
 
 import 'colors.dart';
 import 'interface_level.dart';
@@ -104,10 +102,9 @@ mixin CupertinoRouteTransitionMixin<T> on PageRoute<T> {
   /// [CupertinoSliverNavigationBar]'s `middle`/`largeTitle` widgets when
   /// one is not manually supplied.
   /// {@endtemplate}
-  String get title;
+  String? get title;
 
-
-  ValueNotifier<String> _previousTitle;
+  ValueNotifier<String?>? _previousTitle;
 
   /// The title string of the previous [CupertinoPageRoute].
   ///
@@ -123,23 +120,23 @@ mixin CupertinoRouteTransitionMixin<T> on PageRoute<T> {
   ///
   ///  * [ValueListenableBuilder], which can be used to listen and rebuild
   ///    widgets based on a ValueListenable.
-  ValueListenable<String> get previousTitle {
+  ValueListenable<String?> get previousTitle {
     assert(
       _previousTitle != null,
       'Cannot read the previousTitle for a route that has not yet been installed',
     );
-    return _previousTitle;
+    return _previousTitle!;
   }
 
   @override
-  void didChangePrevious(Route<dynamic> previousRoute) {
-    final String previousTitleString = previousRoute is CupertinoRouteTransitionMixin
+  void didChangePrevious(Route<dynamic>? previousRoute) {
+    final String? previousTitleString = previousRoute is CupertinoRouteTransitionMixin
       ? previousRoute.title
       : null;
     if (_previousTitle == null) {
-      _previousTitle = ValueNotifier<String>(previousTitleString);
+      _previousTitle = ValueNotifier<String?>(previousTitleString);
     } else {
-      _previousTitle.value = previousTitleString;
+      _previousTitle!.value = previousTitleString;
     }
     super.didChangePrevious(previousRoute);
   }
@@ -149,10 +146,10 @@ mixin CupertinoRouteTransitionMixin<T> on PageRoute<T> {
   Duration get transitionDuration => const Duration(milliseconds: 400);
 
   @override
-  Color get barrierColor => null;
+  Color? get barrierColor => null;
 
   @override
-  String get barrierLabel => null;
+  String? get barrierLabel => null;
 
   @override
   bool canTransitionTo(TransitionRoute<dynamic> nextRoute) {
@@ -169,7 +166,7 @@ mixin CupertinoRouteTransitionMixin<T> on PageRoute<T> {
   ///  * [popGestureEnabled], which returns true if a user-triggered pop gesture
   ///    would be allowed.
   static bool isPopGestureInProgress(PageRoute<dynamic> route) {
-    return route.navigator.userGestureInProgress;
+    return route.navigator!.userGestureInProgress;
   }
 
   /// True if an iOS-style back swipe pop gesture is currently underway for this route.
@@ -210,12 +207,12 @@ mixin CupertinoRouteTransitionMixin<T> on PageRoute<T> {
     if (route.fullscreenDialog)
       return false;
     // If we're in an animation already, we cannot be manually swiped.
-    if (route.animation.status != AnimationStatus.completed)
+    if (route.animation!.status != AnimationStatus.completed)
       return false;
     // If we're being popped into, we also cannot be swiped until the pop above
     // it completes. This translates to our secondary animation being
     // dismissed.
-    if (route.secondaryAnimation.status != AnimationStatus.dismissed)
+    if (route.secondaryAnimation!.status != AnimationStatus.dismissed)
       return false;
     // If we're in a gesture already, we cannot start another.
     if (isPopGestureInProgress(route))
@@ -234,7 +231,10 @@ mixin CupertinoRouteTransitionMixin<T> on PageRoute<T> {
       child: child,
     );
     assert(() {
-      if (child == null) {
+      // `child` has a non-nullable return type, but might be null when
+      // running with weak checking, so we need to null check it anyway (and
+      // ignore the warning that the null-handling logic is dead code).
+      if (child == null) { // ignore: dead_code
         throw FlutterError.fromParts(<DiagnosticsNode>[
           ErrorSummary('The builder for route "${settings.name}" returned null.'),
           ErrorDescription('Route builders must never return null.'),
@@ -252,8 +252,8 @@ mixin CupertinoRouteTransitionMixin<T> on PageRoute<T> {
     assert(_isPopGestureEnabled(route));
 
     return _CupertinoBackGestureController<T>(
-      navigator: route.navigator,
-      controller: route.controller, // protected access
+      navigator: route.navigator!,
+      controller: route.controller!, // protected access
     );
   }
 
@@ -340,9 +340,9 @@ class CupertinoPageRoute<T> extends PageRoute<T> with CupertinoRouteTransitionMi
   /// The [builder], [maintainState], and [fullscreenDialog] arguments must not
   /// be null.
   CupertinoPageRoute({
-    @required this.builder,
+    required this.builder,
     this.title,
-    RouteSettings settings,
+    RouteSettings? settings,
     this.maintainState = true,
     bool fullscreenDialog = false,
   }) : assert(builder != null),
@@ -355,7 +355,7 @@ class CupertinoPageRoute<T> extends PageRoute<T> with CupertinoRouteTransitionMi
   final WidgetBuilder builder;
 
   @override
-  final String title;
+  final String? title;
 
   @override
   final bool maintainState;
@@ -370,7 +370,7 @@ class CupertinoPageRoute<T> extends PageRoute<T> with CupertinoRouteTransitionMi
 // the content is up to date after page updates.
 class _PageBasedCupertinoPageRoute<T> extends PageRoute<T> with CupertinoRouteTransitionMixin<T> {
   _PageBasedCupertinoPageRoute({
-    @required CupertinoPage<T> page,
+    required CupertinoPage<T> page,
   }) : assert(page != null),
        assert(opaque),
        super(settings: page);
@@ -381,7 +381,7 @@ class _PageBasedCupertinoPageRoute<T> extends PageRoute<T> with CupertinoRouteTr
   WidgetBuilder get builder => _page.builder;
 
   @override
-  String get title => _page.title;
+  String? get title => _page.title;
 
   @override
   bool get maintainState => _page.maintainState;
@@ -412,13 +412,13 @@ class _PageBasedCupertinoPageRoute<T> extends PageRoute<T> with CupertinoRouteTr
 class CupertinoPage<T> extends Page<T> {
   /// Creates a cupertino page.
   const CupertinoPage({
-    @required this.builder,
+    required this.builder,
     this.maintainState = true,
     this.title,
     this.fullscreenDialog = false,
-    LocalKey key,
-    String name,
-    Object arguments,
+    LocalKey? key,
+    String? name,
+    Object? arguments,
   }) : assert(builder != null),
        assert(maintainState != null),
        assert(fullscreenDialog != null),
@@ -428,7 +428,7 @@ class CupertinoPage<T> extends Page<T> {
   final WidgetBuilder builder;
 
   /// {@macro flutter.cupertino.cupertinoRouteTransitionMixin.title}
-  final String title;
+  final String? title;
 
   /// {@macro flutter.widgets.modalRoute.maintainState}
   final bool maintainState;
@@ -456,11 +456,11 @@ class CupertinoPageTransition extends StatelessWidget {
   ///  * `linearTransition` is whether to perform the transitions linearly.
   ///    Used to precisely track back gesture drags.
   CupertinoPageTransition({
-    Key key,
-    @required Animation<double> primaryRouteAnimation,
-    @required Animation<double> secondaryRouteAnimation,
-    @required this.child,
-    @required bool linearTransition,
+    Key? key,
+    required Animation<double> primaryRouteAnimation,
+    required Animation<double> secondaryRouteAnimation,
+    required this.child,
+    required bool linearTransition,
   }) : assert(linearTransition != null),
        _primaryPositionAnimation =
            (linearTransition
@@ -508,7 +508,7 @@ class CupertinoPageTransition extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     assert(debugCheckHasDirectionality(context));
-    final TextDirection textDirection = Directionality.of(context);
+    final TextDirection? textDirection = Directionality.of(context);
     return SlideTransition(
       position: _secondaryPositionAnimation,
       textDirection: textDirection,
@@ -539,11 +539,11 @@ class CupertinoFullscreenDialogTransition extends StatelessWidget {
   ///  * `linearTransition` is whether to perform the secondary transition linearly.
   ///    Used to precisely track back gesture drags.
   CupertinoFullscreenDialogTransition({
-    Key key,
-    @required Animation<double> primaryRouteAnimation,
-    @required Animation<double> secondaryRouteAnimation,
-    @required this.child,
-    @required bool linearTransition,
+    Key? key,
+    required Animation<double> primaryRouteAnimation,
+    required Animation<double> secondaryRouteAnimation,
+    required this.child,
+    required bool linearTransition,
   }) : _positionAnimation = CurvedAnimation(
          parent: primaryRouteAnimation,
          curve: Curves.linearToEaseOut,
@@ -572,7 +572,7 @@ class CupertinoFullscreenDialogTransition extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     assert(debugCheckHasDirectionality(context));
-    final TextDirection textDirection = Directionality.of(context);
+    final TextDirection? textDirection = Directionality.of(context);
     return SlideTransition(
       position: _secondaryPositionAnimation,
       textDirection: textDirection,
@@ -598,10 +598,10 @@ class CupertinoFullscreenDialogTransition extends StatelessWidget {
 /// detector is associated.
 class _CupertinoBackGestureDetector<T> extends StatefulWidget {
   const _CupertinoBackGestureDetector({
-    Key key,
-    @required this.enabledCallback,
-    @required this.onStartPopGesture,
-    @required this.child,
+    Key? key,
+    required this.enabledCallback,
+    required this.onStartPopGesture,
+    required this.child,
   }) : assert(enabledCallback != null),
        assert(onStartPopGesture != null),
        assert(child != null),
@@ -618,9 +618,9 @@ class _CupertinoBackGestureDetector<T> extends StatefulWidget {
 }
 
 class _CupertinoBackGestureDetectorState<T> extends State<_CupertinoBackGestureDetector<T>> {
-  _CupertinoBackGestureController<T> _backGestureController;
+  _CupertinoBackGestureController<T>? _backGestureController;
 
-  HorizontalDragGestureRecognizer _recognizer;
+  late HorizontalDragGestureRecognizer _recognizer;
 
   @override
   void initState() {
@@ -647,13 +647,13 @@ class _CupertinoBackGestureDetectorState<T> extends State<_CupertinoBackGestureD
   void _handleDragUpdate(DragUpdateDetails details) {
     assert(mounted);
     assert(_backGestureController != null);
-    _backGestureController.dragUpdate(_convertToLogical(details.primaryDelta / context.size.width));
+    _backGestureController!.dragUpdate(_convertToLogical(details.primaryDelta! / context.size!.width));
   }
 
   void _handleDragEnd(DragEndDetails details) {
     assert(mounted);
     assert(_backGestureController != null);
-    _backGestureController.dragEnd(_convertToLogical(details.velocity.pixelsPerSecond.dx / context.size.width));
+    _backGestureController!.dragEnd(_convertToLogical(details.velocity.pixelsPerSecond.dx / context.size!.width));
     _backGestureController = null;
   }
 
@@ -671,13 +671,12 @@ class _CupertinoBackGestureDetectorState<T> extends State<_CupertinoBackGestureD
   }
 
   double _convertToLogical(double value) {
-    switch (Directionality.of(context)) {
+    switch (Directionality.of(context)!) {
       case TextDirection.rtl:
         return -value;
       case TextDirection.ltr:
         return value;
     }
-    return null;
   }
 
   @override
@@ -686,8 +685,8 @@ class _CupertinoBackGestureDetectorState<T> extends State<_CupertinoBackGestureD
     // For devices with notches, the drag area needs to be larger on the side
     // that has the notch.
     double dragAreaWidth = Directionality.of(context) == TextDirection.ltr ?
-                           MediaQuery.of(context).padding.left :
-                           MediaQuery.of(context).padding.right;
+                           MediaQuery.of(context)!.padding.left :
+                           MediaQuery.of(context)!.padding.right;
     dragAreaWidth = max(dragAreaWidth, _kBackGestureWidth);
     return Stack(
       fit: StackFit.passthrough,
@@ -725,8 +724,8 @@ class _CupertinoBackGestureController<T> {
   ///
   /// The [navigator] and [controller] arguments must not be null.
   _CupertinoBackGestureController({
-    @required this.navigator,
-    @required this.controller,
+    required this.navigator,
+    required this.controller,
   }) : assert(navigator != null),
        assert(controller != null) {
     navigator.didStartUserGesture();
@@ -766,7 +765,7 @@ class _CupertinoBackGestureController<T> {
       // We want to cap the animation time, but we want to use a linear curve
       // to determine it.
       final int droppedPageForwardAnimationTime = min(
-        lerpDouble(_kMaxDroppedSwipePageForwardAnimationTime, 0, controller.value).floor(),
+        lerpDouble(_kMaxDroppedSwipePageForwardAnimationTime, 0, controller.value)!.floor(),
         _kMaxPageBackAnimationTime,
       );
       controller.animateTo(1.0, duration: Duration(milliseconds: droppedPageForwardAnimationTime), curve: animationCurve);
@@ -777,7 +776,7 @@ class _CupertinoBackGestureController<T> {
       // The popping may have finished inline if already at the target destination.
       if (controller.isAnimating) {
         // Otherwise, use a custom popping animation duration and curve.
-        final int droppedPageBackAnimationTime = lerpDouble(0, _kMaxDroppedSwipePageForwardAnimationTime, controller.value).floor();
+        final int droppedPageBackAnimationTime = lerpDouble(0, _kMaxDroppedSwipePageForwardAnimationTime, controller.value)!.floor();
         controller.animateBack(0.0, duration: Duration(milliseconds: droppedPageBackAnimationTime), curve: animationCurve);
       }
     }
@@ -786,7 +785,7 @@ class _CupertinoBackGestureController<T> {
       // Keep the userGestureInProgress in true state so we don't change the
       // curve of the page transition mid-flight since CupertinoPageTransition
       // depends on userGestureInProgress.
-      AnimationStatusListener animationStatusCallback;
+      late AnimationStatusListener animationStatusCallback;
       animationStatusCallback = (AnimationStatus status) {
         navigator.didStopUserGesture();
         controller.removeStatusListener(animationStatusCallback);
@@ -818,7 +817,7 @@ class _CupertinoEdgeShadowDecoration extends Decoration {
   // A gradient to draw to the left of the box being decorated.
   // Alignments are relative to the original box translated one box
   // width to the left.
-  final LinearGradient edgeGradient;
+  final LinearGradient? edgeGradient;
 
   // Linearly interpolate between two edge shadow decorations decorations.
   //
@@ -837,9 +836,9 @@ class _CupertinoEdgeShadowDecoration extends Decoration {
   // See also:
   //
   //  * [Decoration.lerp].
-  static _CupertinoEdgeShadowDecoration lerp(
-    _CupertinoEdgeShadowDecoration a,
-    _CupertinoEdgeShadowDecoration b,
+  static _CupertinoEdgeShadowDecoration? lerp(
+    _CupertinoEdgeShadowDecoration? a,
+    _CupertinoEdgeShadowDecoration? b,
     double t,
   ) {
     assert(t != null);
@@ -851,21 +850,21 @@ class _CupertinoEdgeShadowDecoration extends Decoration {
   }
 
   @override
-  _CupertinoEdgeShadowDecoration lerpFrom(Decoration a, double t) {
+  _CupertinoEdgeShadowDecoration lerpFrom(Decoration? a, double t) {
     if (a is _CupertinoEdgeShadowDecoration)
-      return _CupertinoEdgeShadowDecoration.lerp(a, this, t);
-    return _CupertinoEdgeShadowDecoration.lerp(null, this, t);
+      return _CupertinoEdgeShadowDecoration.lerp(a, this, t)!;
+    return _CupertinoEdgeShadowDecoration.lerp(null, this, t)!;
   }
 
   @override
-  _CupertinoEdgeShadowDecoration lerpTo(Decoration b, double t) {
+  _CupertinoEdgeShadowDecoration lerpTo(Decoration? b, double t) {
     if (b is _CupertinoEdgeShadowDecoration)
-      return _CupertinoEdgeShadowDecoration.lerp(this, b, t);
-    return _CupertinoEdgeShadowDecoration.lerp(this, null, t);
+      return _CupertinoEdgeShadowDecoration.lerp(this, b, t)!;
+    return _CupertinoEdgeShadowDecoration.lerp(this, null, t)!;
   }
 
   @override
-  _CupertinoEdgeShadowPainter createBoxPainter([ VoidCallback onChanged ]) {
+  _CupertinoEdgeShadowPainter createBoxPainter([ VoidCallback? onChanged ]) {
     return _CupertinoEdgeShadowPainter(this, onChanged);
   }
 
@@ -891,7 +890,7 @@ class _CupertinoEdgeShadowDecoration extends Decoration {
 class _CupertinoEdgeShadowPainter extends BoxPainter {
   _CupertinoEdgeShadowPainter(
     this._decoration,
-    VoidCallback onChange,
+    VoidCallback? onChange,
   ) : assert(_decoration != null),
       super(onChange);
 
@@ -899,23 +898,23 @@ class _CupertinoEdgeShadowPainter extends BoxPainter {
 
   @override
   void paint(Canvas canvas, Offset offset, ImageConfiguration configuration) {
-    final LinearGradient gradient = _decoration.edgeGradient;
+    final LinearGradient? gradient = _decoration.edgeGradient;
     if (gradient == null)
       return;
     // The drawable space for the gradient is a rect with the same size as
     // its parent box one box width on the start side of the box.
-    final TextDirection textDirection = configuration.textDirection;
+    final TextDirection? textDirection = configuration.textDirection;
     assert(textDirection != null);
     double deltaX;
-    switch (textDirection) {
+    switch (textDirection!) {
       case TextDirection.rtl:
-        deltaX = configuration.size.width;
+        deltaX = configuration.size!.width;
         break;
       case TextDirection.ltr:
-        deltaX = -configuration.size.width;
+        deltaX = -configuration.size!.width;
         break;
     }
-    final Rect rect = (offset & configuration.size).translate(deltaX, 0.0);
+    final Rect rect = (offset & configuration.size!).translate(deltaX, 0.0);
     final Paint paint = Paint()
       ..shader = gradient.createShader(rect, textDirection: textDirection);
 
@@ -925,12 +924,12 @@ class _CupertinoEdgeShadowPainter extends BoxPainter {
 
 class _CupertinoModalPopupRoute<T> extends PopupRoute<T> {
   _CupertinoModalPopupRoute({
-    this.barrierColor,
-    this.barrierLabel,
-    this.builder,
-    bool semanticsDismissible,
-    ImageFilter filter,
-    RouteSettings settings,
+    required this.barrierColor,
+    required this.barrierLabel,
+    required this.builder,
+    bool? semanticsDismissible,
+    required ImageFilter? filter,
+    RouteSettings? settings,
   }) : super(
          filter: filter,
          settings: settings,
@@ -939,13 +938,13 @@ class _CupertinoModalPopupRoute<T> extends PopupRoute<T> {
   }
 
   final WidgetBuilder builder;
-  bool _semanticsDismissible;
+  bool? _semanticsDismissible;
 
   @override
   final String barrierLabel;
 
   @override
-  final Color barrierColor;
+  final Color? barrierColor;
 
   @override
   bool get barrierDismissible => true;
@@ -956,9 +955,9 @@ class _CupertinoModalPopupRoute<T> extends PopupRoute<T> {
   @override
   Duration get transitionDuration => _kModalPopupTransitionDuration;
 
-  Animation<double> _animation;
+  Animation<double>? _animation;
 
-  Tween<Offset> _offsetTween;
+  late Tween<Offset> _offsetTween;
 
   @override
   Animation<double> createAnimation() {
@@ -975,7 +974,7 @@ class _CupertinoModalPopupRoute<T> extends PopupRoute<T> {
       begin: const Offset(0.0, 1.0),
       end: const Offset(0.0, 0.0),
     );
-    return _animation;
+    return _animation!;
   }
 
   @override
@@ -991,7 +990,7 @@ class _CupertinoModalPopupRoute<T> extends PopupRoute<T> {
     return Align(
       alignment: Alignment.bottomCenter,
       child: FractionalTranslation(
-        translation: _offsetTween.evaluate(_animation),
+        translation: _offsetTween.evaluate(_animation!),
         child: child,
       ),
     );
@@ -1030,14 +1029,14 @@ class _CupertinoModalPopupRoute<T> extends PopupRoute<T> {
 ///    `builder` argument to [showCupertinoModalPopup].
 ///  * <https://developer.apple.com/design/human-interface-guidelines/ios/views/action-sheets/>
 Future<T> showCupertinoModalPopup<T>({
-  @required BuildContext context,
-  @required WidgetBuilder builder,
-  ImageFilter filter,
+  required BuildContext context,
+  required WidgetBuilder builder,
+  ImageFilter? filter,
   bool useRootNavigator = true,
-  bool semanticsDismissible,
+  bool? semanticsDismissible,
 }) {
   assert(useRootNavigator != null);
-  return Navigator.of(context, rootNavigator: useRootNavigator).push(
+  return Navigator.of(context, rootNavigator: useRootNavigator)!.push(
     _CupertinoModalPopupRoute<T>(
       barrierColor: CupertinoDynamicColor.resolve(_kModalBarrierColor, context),
       barrierLabel: 'Dismiss',
@@ -1110,19 +1109,19 @@ Widget _buildCupertinoDialogTransitions(BuildContext context, Animation<double>
 ///  * [showGeneralDialog], which allows for customization of the dialog popup.
 ///  * <https://developer.apple.com/ios/human-interface-guidelines/views/alerts/>
 Future<T> showCupertinoDialog<T>({
-  @required BuildContext context,
-  @required WidgetBuilder builder,
+  required BuildContext context,
+  required WidgetBuilder builder,
   bool useRootNavigator = true,
   bool barrierDismissible = false,
-  RouteSettings routeSettings,
+  RouteSettings? routeSettings,
 }) {
   assert(builder != null);
   assert(useRootNavigator != null);
   return showGeneralDialog(
     context: context,
     barrierDismissible: barrierDismissible,
-    barrierLabel: CupertinoLocalizations.of(context).modalBarrierDismissLabel,
-    barrierColor: CupertinoDynamicColor.resolve(_kModalBarrierColor, context),
+    barrierLabel: CupertinoLocalizations.of(context)!.modalBarrierDismissLabel,
+    barrierColor: CupertinoDynamicColor.resolve(_kModalBarrierColor, context)!,
     // This transition duration was eyeballed comparing with iOS
     transitionDuration: const Duration(milliseconds: 250),
     pageBuilder: (BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {

packages/flutter/lib/src/cupertino/scrollbar.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/gestures.dart';
@@ -56,14 +54,14 @@ class CupertinoScrollbar extends StatefulWidget {
   /// The [child] should be a source of [ScrollNotification] notifications,
   /// typically a [Scrollable] widget.
   const CupertinoScrollbar({
-    Key key,
+    Key? key,
     this.controller,
     this.isAlwaysShown = false,
     this.thickness = defaultThickness,
     this.thicknessWhileDragging = defaultThicknessWhileDragging,
     this.radius = defaultRadius,
     this.radiusWhileDragging = defaultRadiusWhileDragging,
-    @required this.child,
+    required this.child,
   }) : assert(thickness != null),
        assert(thickness < double.infinity),
        assert(thicknessWhileDragging != null),
@@ -145,7 +143,7 @@ class CupertinoScrollbar extends StatefulWidget {
   /// ```
   /// {@end-tool}
   /// {@endtemplate}
-  final ScrollController controller;
+  final ScrollController? controller;
 
   /// {@template flutter.cupertino.cupertinoScrollbar.isAlwaysShown}
   /// Indicates whether the [Scrollbar] should always be visible.
@@ -237,25 +235,25 @@ class CupertinoScrollbar extends StatefulWidget {
 
 class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProviderStateMixin {
   final GlobalKey _customPaintKey = GlobalKey();
-  ScrollbarPainter _painter;
+  ScrollbarPainter? _painter;
 
-  AnimationController _fadeoutAnimationController;
-  Animation<double> _fadeoutOpacityAnimation;
-  AnimationController _thicknessAnimationController;
-  Timer _fadeoutTimer;
-  double _dragScrollbarPositionY;
-  Drag _drag;
+  late AnimationController _fadeoutAnimationController;
+  late Animation<double> _fadeoutOpacityAnimation;
+  late AnimationController _thicknessAnimationController;
+  Timer? _fadeoutTimer;
+  double? _dragScrollbarPositionY;
+  Drag? _drag;
 
   double get _thickness {
     return widget.thickness + _thicknessAnimationController.value * (widget.thicknessWhileDragging - widget.thickness);
   }
 
   Radius get _radius {
-    return Radius.lerp(widget.radius, widget.radiusWhileDragging, _thicknessAnimationController.value);
+    return Radius.lerp(widget.radius, widget.radiusWhileDragging, _thicknessAnimationController.value)!;
   }
 
-  ScrollController _currentController;
-  ScrollController get _controller =>
+  ScrollController? _currentController;
+  ScrollController? get _controller =>
       widget.controller ?? PrimaryScrollController.of(context);
 
   @override
@@ -274,7 +272,7 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
       duration: _kScrollbarResizeDuration,
     );
     _thicknessAnimationController.addListener(() {
-      _painter.updateThickness(_thickness, _radius);
+      _painter!.updateThickness(_thickness, _radius);
     });
   }
 
@@ -284,10 +282,10 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
     if (_painter == null) {
       _painter = _buildCupertinoScrollbarPainter(context);
     } else {
-      _painter
-        ..textDirection = Directionality.of(context)
-        ..color = CupertinoDynamicColor.resolve(_kScrollbarColor, context)
-        ..padding = MediaQuery.of(context).padding;
+      _painter!
+        ..textDirection = Directionality.of(context)!
+        ..color = CupertinoDynamicColor.resolve(_kScrollbarColor, context)!
+        ..padding = MediaQuery.of(context)!.padding;
     }
     _triggerScrollbar();
   }
@@ -296,7 +294,7 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
   void didUpdateWidget(CupertinoScrollbar oldWidget) {
     super.didUpdateWidget(oldWidget);
     assert(_painter != null);
-    _painter.updateThickness(_thickness, _radius);
+    _painter!.updateThickness(_thickness, _radius);
     if (widget.isAlwaysShown != oldWidget.isAlwaysShown) {
       if (widget.isAlwaysShown == true) {
         _triggerScrollbar();
@@ -310,14 +308,14 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
   /// Returns a [ScrollbarPainter] visually styled like the iOS scrollbar.
   ScrollbarPainter _buildCupertinoScrollbarPainter(BuildContext context) {
     return ScrollbarPainter(
-      color: CupertinoDynamicColor.resolve(_kScrollbarColor, context),
-      textDirection: Directionality.of(context),
+      color: CupertinoDynamicColor.resolve(_kScrollbarColor, context)!,
+      textDirection: Directionality.of(context)!,
       thickness: _thickness,
       fadeoutOpacityAnimation: _fadeoutOpacityAnimation,
       mainAxisMargin: _kScrollbarMainAxisMargin,
       crossAxisMargin: _kScrollbarCrossAxisMargin,
       radius: _radius,
-      padding: MediaQuery.of(context).padding,
+      padding: MediaQuery.of(context)!.padding,
       minLength: _kScrollbarMinLength,
       minOverscrollLength: _kScrollbarMinOverscrollLength,
     );
@@ -327,10 +325,10 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
   // show immediately when isAlwaysShown is true.  A scroll event is required in
   // order to paint the thumb.
   void _triggerScrollbar() {
-    WidgetsBinding.instance.addPostFrameCallback((Duration duration) {
+    WidgetsBinding.instance!.addPostFrameCallback((Duration duration) {
       if (widget.isAlwaysShown) {
         _fadeoutTimer?.cancel();
-        widget.controller.position.didUpdateScrollPositionBy(0);
+        widget.controller!.position.didUpdateScrollPositionBy(0);
       }
     });
   }
@@ -342,18 +340,18 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
     // Convert primaryDelta, the amount that the scrollbar moved since the last
     // time _dragScrollbar was called, into the coordinate space of the scroll
     // position, and create/update the drag event with that position.
-    final double scrollOffsetLocal = _painter.getTrackToScroll(primaryDelta);
-    final double scrollOffsetGlobal = scrollOffsetLocal + _currentController.position.pixels;
+    final double scrollOffsetLocal = _painter!.getTrackToScroll(primaryDelta);
+    final double scrollOffsetGlobal = scrollOffsetLocal + _currentController!.position.pixels;
 
     if (_drag == null) {
-      _drag = _currentController.position.drag(
+      _drag = _currentController!.position.drag(
         DragStartDetails(
           globalPosition: Offset(0.0, scrollOffsetGlobal),
         ),
         () {},
       );
     } else {
-      _drag.update(DragUpdateDetails(
+      _drag!.update(DragUpdateDetails(
         globalPosition: Offset(0.0, scrollOffsetGlobal),
         delta: Offset(0.0, -scrollOffsetLocal),
         primaryDelta: -scrollOffsetLocal,
@@ -373,7 +371,7 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
 
   bool _checkVertical() {
     try {
-      return _currentController.position.axis == Axis.vertical;
+      return _currentController!.position.axis == Axis.vertical;
     } catch (_) {
       // Ignore the gesture if we cannot determine the direction.
       return false;
@@ -410,7 +408,7 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
     if (!_checkVertical()) {
       return;
     }
-    _dragScrollbar(details.localPosition.dy - _dragScrollbarPositionY);
+    _dragScrollbar(details.localPosition.dy - _dragScrollbarPositionY!);
     _dragScrollbarPositionY = details.localPosition.dy;
   }
 
@@ -430,7 +428,7 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
     _startFadeoutTimer();
     _thicknessAnimationController.reverse();
     _dragScrollbarPositionY = null;
-    final double scrollVelocityY = _painter.getTrackToScroll(trackVelocityY);
+    final double scrollVelocityY = _painter!.getTrackToScroll(trackVelocityY);
     _drag?.end(DragEndDetails(
       primaryVelocity: -scrollVelocityY,
       velocity: Velocity(
@@ -457,7 +455,7 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
       }
 
       _fadeoutTimer?.cancel();
-      _painter.update(notification.metrics, notification.metrics.axisDirection);
+      _painter!.update(notification.metrics, notification.metrics.axisDirection);
     } else if (notification is ScrollEndNotification) {
       // On iOS, the scrollbar can only go away once the user lifted the finger.
       if (_dragScrollbarPositionY == null) {
@@ -496,7 +494,7 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
     _fadeoutAnimationController.dispose();
     _thicknessAnimationController.dispose();
     _fadeoutTimer?.cancel();
-    _painter.dispose();
+    _painter!.dispose();
     super.dispose();
   }
 
@@ -522,10 +520,10 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
 // thumb and ignores everything else.
 class _ThumbPressGestureRecognizer extends LongPressGestureRecognizer {
   _ThumbPressGestureRecognizer({
-    double postAcceptSlopTolerance,
-    PointerDeviceKind kind,
-    Object debugOwner,
-    GlobalKey customPaintKey,
+    double? postAcceptSlopTolerance,
+    PointerDeviceKind? kind,
+    required Object debugOwner,
+    required GlobalKey customPaintKey,
   }) :  _customPaintKey = customPaintKey,
         super(
           postAcceptSlopTolerance: postAcceptSlopTolerance,
@@ -552,9 +550,9 @@ bool _hitTestInteractive(GlobalKey customPaintKey, Offset offset) {
   if (customPaintKey.currentContext == null) {
     return false;
   }
-  final CustomPaint customPaint = customPaintKey.currentContext.widget as CustomPaint;
+  final CustomPaint customPaint = customPaintKey.currentContext!.widget as CustomPaint;
   final ScrollbarPainter painter = customPaint.foregroundPainter as ScrollbarPainter;
-  final RenderBox renderBox = customPaintKey.currentContext.findRenderObject() as RenderBox;
+  final RenderBox renderBox = customPaintKey.currentContext!.findRenderObject() as RenderBox;
   final Offset localOffset = renderBox.globalToLocal(offset);
   return painter.hitTestInteractive(localOffset);
 }

packages/flutter/lib/src/cupertino/switch.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 lerpDouble;
 
 import 'package:flutter/foundation.dart';
@@ -58,9 +56,9 @@ class CupertinoSwitch extends StatefulWidget {
   /// The [value] parameter must not be null.
   /// The [dragStartBehavior] parameter defaults to [DragStartBehavior.start] and must not be null.
   const CupertinoSwitch({
-    Key key,
-    @required this.value,
-    @required this.onChanged,
+    Key? key,
+    required this.value,
+    required this.onChanged,
     this.activeColor,
     this.trackColor,
     this.dragStartBehavior = DragStartBehavior.start,
@@ -95,18 +93,18 @@ class CupertinoSwitch extends StatefulWidget {
   ///   },
   /// )
   /// ```
-  final ValueChanged<bool> onChanged;
+  final ValueChanged<bool>? onChanged;
 
   /// The color to use when this switch is on.
   ///
   /// Defaults to [CupertinoColors.systemGreen] when null and ignores
   /// the [CupertinoTheme] in accordance to native iOS behavior.
-  final Color activeColor;
+  final Color? activeColor;
 
   /// The color to use for the background when the switch is off.
   ///
   /// Defaults to [CupertinoColors.secondarySystemFill] when null.
-  final Color trackColor;
+  final Color? trackColor;
 
   /// {@template flutter.cupertino.switch.dragStartBehavior}
   /// Determines the way that drag start behavior is handled.
@@ -142,14 +140,14 @@ class CupertinoSwitch extends StatefulWidget {
 }
 
 class _CupertinoSwitchState extends State<CupertinoSwitch> with TickerProviderStateMixin {
-  TapGestureRecognizer _tap;
-  HorizontalDragGestureRecognizer _drag;
+  late TapGestureRecognizer _tap;
+  late HorizontalDragGestureRecognizer _drag;
 
-  AnimationController _positionController;
-  CurvedAnimation position;
+  late AnimationController _positionController;
+  late CurvedAnimation position;
 
-  AnimationController _reactionController;
-  Animation<double> _reaction;
+  late AnimationController _reactionController;
+  late Animation<double> _reaction;
 
   bool get isInteractive => widget.onChanged != null;
 
@@ -209,8 +207,8 @@ class _CupertinoSwitchState extends State<CupertinoSwitch> with TickerProviderSt
   void _resumePositionAnimation({ bool isLinear = true }) {
     needsPositionAnimation = false;
     position
-      ..curve = isLinear ? null : Curves.ease
-      ..reverseCurve = isLinear ? null : Curves.ease.flipped;
+      ..curve = isLinear ? Curves.linear : Curves.ease
+      ..reverseCurve = isLinear ? Curves.linear : Curves.ease.flipped;
     if (widget.value)
       _positionController.forward();
     else
@@ -225,7 +223,7 @@ class _CupertinoSwitchState extends State<CupertinoSwitch> with TickerProviderSt
 
   void _handleTap() {
     if (isInteractive) {
-      widget.onChanged(!widget.value);
+      widget.onChanged!(!widget.value);
       _emitVibration();
     }
   }
@@ -253,10 +251,10 @@ class _CupertinoSwitchState extends State<CupertinoSwitch> with TickerProviderSt
   void _handleDragUpdate(DragUpdateDetails details) {
     if (isInteractive) {
       position
-        ..curve = null
-        ..reverseCurve = null;
-      final double delta = details.primaryDelta / _kTrackInnerLength;
-      switch (Directionality.of(context)) {
+        ..curve = Curves.linear
+        ..reverseCurve = Curves.linear;
+      final double delta = details.primaryDelta! / _kTrackInnerLength;
+      switch (Directionality.of(context)!) {
         case TextDirection.rtl:
           _positionController.value -= delta;
           break;
@@ -272,7 +270,7 @@ class _CupertinoSwitchState extends State<CupertinoSwitch> with TickerProviderSt
     setState(() { needsPositionAnimation = true; });
     // Call onChanged when the user's intent to change value is clear.
     if (position.value >= 0.5 != widget.value)
-      widget.onChanged(!widget.value);
+      widget.onChanged!(!widget.value);
     _reactionController.reverse();
   }
 
@@ -301,10 +299,10 @@ class _CupertinoSwitchState extends State<CupertinoSwitch> with TickerProviderSt
         activeColor: CupertinoDynamicColor.resolve(
           widget.activeColor ?? CupertinoColors.systemGreen,
           context,
-        ),
-        trackColor: CupertinoDynamicColor.resolve(widget.trackColor ?? CupertinoColors.secondarySystemFill, context),
+        )!,
+        trackColor: CupertinoDynamicColor.resolve(widget.trackColor ?? CupertinoColors.secondarySystemFill, context)!,
         onChanged: widget.onChanged,
-        textDirection: Directionality.of(context),
+        textDirection: Directionality.of(context)!,
         state: this,
       ),
     );
@@ -323,19 +321,19 @@ class _CupertinoSwitchState extends State<CupertinoSwitch> with TickerProviderSt
 
 class _CupertinoSwitchRenderObjectWidget extends LeafRenderObjectWidget {
   const _CupertinoSwitchRenderObjectWidget({
-    Key key,
-    this.value,
-    this.activeColor,
-    this.trackColor,
-    this.onChanged,
-    this.textDirection,
-    this.state,
+    Key? key,
+    required this.value,
+    required this.activeColor,
+    required this.trackColor,
+    required this.onChanged,
+    required this.textDirection,
+    required this.state,
   }) : super(key: key);
 
   final bool value;
   final Color activeColor;
   final Color trackColor;
-  final ValueChanged<bool> onChanged;
+  final ValueChanged<bool>? onChanged;
   final _CupertinoSwitchState state;
   final TextDirection textDirection;
 
@@ -378,12 +376,12 @@ const Duration _kToggleDuration = Duration(milliseconds: 200);
 
 class _RenderCupertinoSwitch extends RenderConstrainedBox {
   _RenderCupertinoSwitch({
-    @required bool value,
-    @required Color activeColor,
-    @required Color trackColor,
-    ValueChanged<bool> onChanged,
-    @required TextDirection textDirection,
-    @required _CupertinoSwitchState state,
+    required bool value,
+    required Color activeColor,
+    required Color trackColor,
+    ValueChanged<bool>? onChanged,
+    required TextDirection textDirection,
+    required _CupertinoSwitchState state,
   }) : assert(value != null),
        assert(activeColor != null),
        assert(state != null),
@@ -430,9 +428,9 @@ class _RenderCupertinoSwitch extends RenderConstrainedBox {
     markNeedsPaint();
   }
 
-  ValueChanged<bool> get onChanged => _onChanged;
-  ValueChanged<bool> _onChanged;
-  set onChanged(ValueChanged<bool> value) {
+  ValueChanged<bool>? get onChanged => _onChanged;
+  ValueChanged<bool>? _onChanged;
+  set onChanged(ValueChanged<bool>? value) {
     if (value == _onChanged)
       return;
     final bool wasInteractive = isInteractive;
@@ -496,7 +494,7 @@ class _RenderCupertinoSwitch extends RenderConstrainedBox {
     }
 
     final Paint paint = Paint()
-      ..color = Color.lerp(trackColor, activeColor, currentValue);
+      ..color = Color.lerp(trackColor, activeColor, currentValue)!;
 
     final Rect trackRect = Rect.fromLTWH(
         offset.dx + (size.width - _kTrackWidth) / 2.0,
@@ -512,12 +510,12 @@ class _RenderCupertinoSwitch extends RenderConstrainedBox {
       trackRect.left + _kTrackInnerStart - CupertinoThumbPainter.radius,
       trackRect.left + _kTrackInnerEnd - CupertinoThumbPainter.radius - currentThumbExtension,
       visualPosition,
-    );
+    )!;
     final double thumbRight = lerpDouble(
       trackRect.left + _kTrackInnerStart + CupertinoThumbPainter.radius + currentThumbExtension,
       trackRect.left + _kTrackInnerEnd + CupertinoThumbPainter.radius,
       visualPosition,
-    );
+    )!;
     final double thumbCenterY = offset.dy + size.height / 2.0;
     final Rect thumbBounds = Rect.fromLTRB(
       thumbLeft,