migrate some cupertino files to nullsafety (#66424)

* migrate some cupertino files to nullsafety

* address review comments

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_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/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/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,