migrate cupertino to nullsafety (#66493)

packages/flutter/lib/cupertino.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
-
 /// Flutter widgets implementing the current iOS design language.
 ///
 /// To use, import `package:flutter/cupertino.dart`.

packages/flutter/lib/src/cupertino/nav_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:math' as math;
 import 'dart:ui' show ImageFilter;
 
@@ -58,7 +56,7 @@ const _HeroTag _defaultHeroTag = _HeroTag(null);
 class _HeroTag {
   const _HeroTag(this.navigator);
 
-  final NavigatorState navigator;
+  final NavigatorState? navigator;
 
   // Let the Hero tag be described in tree dumps.
   @override
@@ -88,10 +86,10 @@ class _HeroTag {
 /// When `updateSystemUiOverlay` is true, the nav bar will update the OS
 /// status bar's color theme based on the background color of the nav bar.
 Widget _wrapWithBackground({
-  Border border,
-  Color backgroundColor,
-  Brightness brightness,
-  Widget child,
+  Border? border,
+  required Color backgroundColor,
+  Brightness? brightness,
+  required Widget child,
   bool updateSystemUiOverlay = true,
 }) {
   Widget result = child;
@@ -137,7 +135,7 @@ Widget _wrapWithBackground({
 // `actionsForegroundColor`. CupertinoThemes can be used to support these
 // scenarios now. To support `actionsForegroundColor`, the nav bar rewraps
 // its children with a CupertinoTheme.
-Widget _wrapActiveColor(Color color, BuildContext context, Widget child) {
+Widget _wrapActiveColor(Color? color, BuildContext context, Widget child) {
   if (color == null) {
     return child;
   }
@@ -150,7 +148,7 @@ Widget _wrapActiveColor(Color color, BuildContext context, Widget child) {
 
 // Whether the current route supports nav bar hero transitions from or to.
 bool _isTransitionable(BuildContext context) {
-  final ModalRoute<dynamic> route = ModalRoute.of(context);
+  final ModalRoute<dynamic>? route = ModalRoute.of(context);
 
   // Fullscreen dialogs never transitions their nav bar with other push-style
   // pages' nav bars or with other fullscreen dialog pages on the way in or on
@@ -209,7 +207,7 @@ bool _isTransitionable(BuildContext context) {
 class CupertinoNavigationBar extends StatefulWidget implements ObstructingPreferredSizeWidget {
   /// Creates a navigation bar in the iOS style.
   const CupertinoNavigationBar({
-    Key key,
+    Key? key,
     this.leading,
     this.automaticallyImplyLeading = true,
     this.automaticallyImplyMiddle = true,
@@ -245,7 +243,7 @@ class CupertinoNavigationBar extends StatefulWidget implements ObstructingPrefer
   /// If null and [automaticallyImplyLeading] is true, an appropriate button
   /// will be automatically created.
   /// {@endtemplate}
-  final Widget leading;
+  final Widget? leading;
 
   /// {@template flutter.cupertino.navBar.automaticallyImplyLeading}
   /// Controls whether we should try to imply the leading widget if null.
@@ -286,7 +284,7 @@ class CupertinoNavigationBar extends StatefulWidget implements ObstructingPrefer
   /// Has no effect when [leading] is not null or if [automaticallyImplyLeading]
   /// is false.
   /// {@endtemplate}
-  final String previousPageTitle;
+  final String? previousPageTitle;
 
   /// Widget to place in the middle of the navigation bar. Normally a title or
   /// a segmented control.
@@ -294,13 +292,13 @@ class CupertinoNavigationBar extends StatefulWidget implements ObstructingPrefer
   /// If null and [automaticallyImplyMiddle] is true, an appropriate [Text]
   /// title will be created if the current route is a [CupertinoPageRoute] and
   /// has a `title`.
-  final Widget middle;
+  final Widget? middle;
 
   /// {@template flutter.cupertino.navBar.trailing}
   /// Widget to place at the end of the navigation bar. Normally additional actions
   /// taken on the page such as a search or edit function.
   /// {@endtemplate}
-  final Widget trailing;
+  final Widget? trailing;
 
   // TODO(xster): https://github.com/flutter/flutter/issues/10469 implement
   // support for double row navigation bars.
@@ -312,7 +310,7 @@ class CupertinoNavigationBar extends StatefulWidget implements ObstructingPrefer
   ///
   /// Defaults to [CupertinoTheme]'s `barBackgroundColor` if null.
   /// {@endtemplate}
-  final Color backgroundColor;
+  final Color? backgroundColor;
 
   /// {@template flutter.cupertino.navBar.brightness}
   /// The brightness of the specified [backgroundColor].
@@ -324,7 +322,7 @@ class CupertinoNavigationBar extends StatefulWidget implements ObstructingPrefer
   /// If set to null, the value of the property will be inferred from the relative
   /// luminance of [backgroundColor].
   /// {@endtemplate}
-  final Brightness brightness;
+  final Brightness? brightness;
 
   /// {@template flutter.cupertino.navBar.padding}
   /// Padding for the contents of the navigation bar.
@@ -339,7 +337,7 @@ class CupertinoNavigationBar extends StatefulWidget implements ObstructingPrefer
   ///
   /// Vertical padding won't change the height of the nav bar.
   /// {@endtemplate}
-  final EdgeInsetsDirectional padding;
+  final EdgeInsetsDirectional? padding;
 
   /// {@template flutter.cupertino.navBar.border}
   /// The border of the navigation bar. By default renders a single pixel bottom border side.
@@ -361,7 +359,7 @@ class CupertinoNavigationBar extends StatefulWidget implements ObstructingPrefer
     'Use CupertinoTheme and primaryColor to propagate color. '
     'This feature was deprecated after v1.1.2.'
   )
-  final Color actionsForegroundColor;
+  final Color? actionsForegroundColor;
 
   /// {@template flutter.cupertino.navBar.transitionBetweenRoutes}
   /// Whether to transition between navigation bars.
@@ -422,7 +420,7 @@ class CupertinoNavigationBar extends StatefulWidget implements ObstructingPrefer
 // don't change when rebuilding the nav bar, causing the sub-components to
 // lose their own states.
 class _CupertinoNavigationBarState extends State<CupertinoNavigationBar> {
-  _NavigationBarStaticComponentsKeys keys;
+  late _NavigationBarStaticComponentsKeys keys;
 
   @override
   void initState() {
@@ -462,7 +460,7 @@ class _CupertinoNavigationBarState extends State<CupertinoNavigationBar> {
       ),
     );
 
-    final Color actionsForegroundColor = CupertinoDynamicColor.resolve(
+    final Color? actionsForegroundColor = CupertinoDynamicColor.resolve(
       widget.actionsForegroundColor,
       context,
     );
@@ -563,7 +561,7 @@ class CupertinoSliverNavigationBar extends StatefulWidget {
   ///
   /// The [largeTitle] argument is required and must not be null.
   const CupertinoSliverNavigationBar({
-    Key key,
+    Key? key,
     this.largeTitle,
     this.leading,
     this.automaticallyImplyLeading = true,
@@ -609,12 +607,12 @@ class CupertinoSliverNavigationBar extends StatefulWidget {
   ///
   /// This parameter must either be non-null or the route must have a title
   /// ([CupertinoPageRoute.title]) and [automaticallyImplyTitle] must be true.
-  final Widget largeTitle;
+  final Widget? largeTitle;
 
   /// {@macro flutter.cupertino.navBar.leading}
   ///
   /// This widget is visible in both collapsed and expanded states.
-  final Widget leading;
+  final Widget? leading;
 
   /// {@macro flutter.cupertino.navBar.automaticallyImplyLeading}
   final bool automaticallyImplyLeading;
@@ -629,7 +627,7 @@ class CupertinoSliverNavigationBar extends StatefulWidget {
   final bool automaticallyImplyTitle;
 
   /// {@macro flutter.cupertino.navBar.previousPageTitle}
-  final String previousPageTitle;
+  final String? previousPageTitle;
 
   /// A widget to place in the middle of the static navigation bar instead of
   /// the [largeTitle].
@@ -637,21 +635,21 @@ class CupertinoSliverNavigationBar extends StatefulWidget {
   /// This widget is visible in both collapsed and expanded states. The text
   /// supplied in [largeTitle] will no longer appear in collapsed state if a
   /// [middle] widget is provided.
-  final Widget middle;
+  final Widget? middle;
 
   /// {@macro flutter.cupertino.navBar.trailing}
   ///
   /// This widget is visible in both collapsed and expanded states.
-  final Widget trailing;
+  final Widget? trailing;
 
   /// {@macro flutter.cupertino.navBar.backgroundColor}
-  final Color backgroundColor;
+  final Color? backgroundColor;
 
   /// {@macro flutter.cupertino.navBar.brightness}
-  final Brightness brightness;
+  final Brightness? brightness;
 
   /// {@macro flutter.cupertino.navBar.padding}
-  final EdgeInsetsDirectional padding;
+  final EdgeInsetsDirectional? padding;
 
   /// {@macro flutter.cupertino.navBar.border}
   final Border border;
@@ -664,7 +662,7 @@ class CupertinoSliverNavigationBar extends StatefulWidget {
     'Use CupertinoTheme and primaryColor to propagate color. '
     'This feature was deprecated after v1.1.2.'
   )
-  final Color actionsForegroundColor;
+  final Color? actionsForegroundColor;
 
   /// {@macro flutter.cupertino.navBar.transitionBetweenRoutes}
   final bool transitionBetweenRoutes;
@@ -673,7 +671,7 @@ class CupertinoSliverNavigationBar extends StatefulWidget {
   final Object heroTag;
 
   /// True if the navigation bar's background color has no transparency.
-  bool get opaque => backgroundColor.alpha == 0xFF;
+  bool get opaque => backgroundColor?.alpha == 0xFF;
 
   @override
   _CupertinoSliverNavigationBarState createState() => _CupertinoSliverNavigationBarState();
@@ -683,7 +681,7 @@ class CupertinoSliverNavigationBar extends StatefulWidget {
 // don't change when rebuilding the nav bar, causing the sub-components to
 // lose their own states.
 class _CupertinoSliverNavigationBarState extends State<CupertinoSliverNavigationBar> {
-  _NavigationBarStaticComponentsKeys keys;
+  late _NavigationBarStaticComponentsKeys keys;
 
   @override
   void initState() {
@@ -716,7 +714,7 @@ class _CupertinoSliverNavigationBarState extends State<CupertinoSliverNavigation
       actionsForegroundColor,
       context,
       MediaQuery(
-        data: MediaQuery.of(context).copyWith(textScaleFactor: 1),
+        data: MediaQuery.of(context)!.copyWith(textScaleFactor: 1),
         child: SliverPersistentHeader(
           pinned: true, // iOS navigation bars are always pinned.
           delegate: _LargeTitleNavigationBarSliverDelegate(
@@ -730,7 +728,7 @@ class _CupertinoSliverNavigationBarState extends State<CupertinoSliverNavigation
             actionsForegroundColor: actionsForegroundColor,
             transitionBetweenRoutes: widget.transitionBetweenRoutes,
             heroTag: widget.heroTag,
-            persistentHeight: _kNavBarPersistentHeight + MediaQuery.of(context).padding.top,
+            persistentHeight: _kNavBarPersistentHeight + MediaQuery.of(context)!.padding.top,
             alwaysShowMiddle: widget.middle != null,
           ),
         ),
@@ -742,29 +740,29 @@ class _CupertinoSliverNavigationBarState extends State<CupertinoSliverNavigation
 class _LargeTitleNavigationBarSliverDelegate
     extends SliverPersistentHeaderDelegate with DiagnosticableTreeMixin {
   _LargeTitleNavigationBarSliverDelegate({
-    @required this.keys,
-    @required this.components,
-    @required this.userMiddle,
-    @required this.backgroundColor,
-    @required this.brightness,
-    @required this.border,
-    @required this.padding,
-    @required this.actionsForegroundColor,
-    @required this.transitionBetweenRoutes,
-    @required this.heroTag,
-    @required this.persistentHeight,
-    @required this.alwaysShowMiddle,
+    required this.keys,
+    required this.components,
+    required this.userMiddle,
+    required this.backgroundColor,
+    required this.brightness,
+    required this.border,
+    required this.padding,
+    required this.actionsForegroundColor,
+    required this.transitionBetweenRoutes,
+    required this.heroTag,
+    required this.persistentHeight,
+    required this.alwaysShowMiddle,
   }) : assert(persistentHeight != null),
        assert(alwaysShowMiddle != null),
        assert(transitionBetweenRoutes != null);
 
   final _NavigationBarStaticComponentsKeys keys;
   final _NavigationBarStaticComponents components;
-  final Widget userMiddle;
+  final Widget? userMiddle;
   final Color backgroundColor;
-  final Brightness brightness;
-  final Border border;
-  final EdgeInsetsDirectional padding;
+  final Brightness? brightness;
+  final Border? border;
+  final EdgeInsetsDirectional? padding;
   final Color actionsForegroundColor;
   final bool transitionBetweenRoutes;
   final Object heroTag;
@@ -792,7 +790,7 @@ class _LargeTitleNavigationBarSliverDelegate
 
     final Widget navBar = _wrapWithBackground(
       border: border,
-      backgroundColor: CupertinoDynamicColor.resolve(backgroundColor, context),
+      backgroundColor: CupertinoDynamicColor.resolve(backgroundColor, context)!,
       brightness: brightness,
       child: DefaultTextStyle(
         style: CupertinoTheme.of(context).textTheme.textStyle,
@@ -829,7 +827,7 @@ class _LargeTitleNavigationBarSliverDelegate
                             style: CupertinoTheme.of(context).textTheme.navLargeTitleTextStyle,
                             maxLines: 1,
                             overflow: TextOverflow.ellipsis,
-                            child: components.largeTitle,
+                            child: components.largeTitle!,
                           ),
                         ),
                       ),
@@ -900,22 +898,22 @@ class _LargeTitleNavigationBarSliverDelegate
 /// doesn't scroll.
 class _PersistentNavigationBar extends StatelessWidget {
   const _PersistentNavigationBar({
-    Key key,
-    this.components,
+    Key? key,
+    required this.components,
     this.padding,
     this.middleVisible,
   }) : super(key: key);
 
   final _NavigationBarStaticComponents components;
 
-  final EdgeInsetsDirectional padding;
+  final EdgeInsetsDirectional? padding;
   /// Whether the middle widget has a visible animated opacity. A null value
   /// means the middle opacity will not be animated.
-  final bool middleVisible;
+  final bool? middleVisible;
 
   @override
   Widget build(BuildContext context) {
-    Widget middle = components.middle;
+    Widget? middle = components.middle;
 
     if (middle != null) {
       middle = DefaultTextStyle(
@@ -927,15 +925,15 @@ class _PersistentNavigationBar extends StatelessWidget {
       middle = middleVisible == null
         ? middle
         : AnimatedOpacity(
-          opacity: middleVisible ? 1.0 : 0.0,
+          opacity: middleVisible! ? 1.0 : 0.0,
           duration: _kNavBarTitleFadeDuration,
           child: middle,
         );
     }
 
-    Widget leading = components.leading;
-    final Widget backChevron = components.backChevron;
-    final Widget backLabel = components.backLabel;
+    Widget? leading = components.leading;
+    final Widget? backChevron = components.backChevron;
+    final Widget? backLabel = components.backLabel;
 
     if (leading == null && backChevron != null && backLabel != null) {
       leading = CupertinoNavigationBarBackButton._assemble(
@@ -955,15 +953,15 @@ class _PersistentNavigationBar extends StatelessWidget {
     if (padding != null) {
       paddedToolbar = Padding(
         padding: EdgeInsets.only(
-          top: padding.top,
-          bottom: padding.bottom,
+          top: padding!.top,
+          bottom: padding!.bottom,
         ),
         child: paddedToolbar,
       );
     }
 
     return SizedBox(
-      height: _kNavBarPersistentHeight + MediaQuery.of(context).padding.top,
+      height: _kNavBarPersistentHeight + MediaQuery.of(context)!.padding.top,
       child: SafeArea(
         bottom: false,
         child: paddedToolbar,
@@ -1005,17 +1003,17 @@ class _NavigationBarStaticComponentsKeys {
 @immutable
 class _NavigationBarStaticComponents {
   _NavigationBarStaticComponents({
-    @required _NavigationBarStaticComponentsKeys keys,
-    @required ModalRoute<dynamic> route,
-    @required Widget userLeading,
-    @required bool automaticallyImplyLeading,
-    @required bool automaticallyImplyTitle,
-    @required String previousPageTitle,
-    @required Widget userMiddle,
-    @required Widget userTrailing,
-    @required Widget userLargeTitle,
-    @required EdgeInsetsDirectional padding,
-    @required bool large,
+    required _NavigationBarStaticComponentsKeys keys,
+    required ModalRoute<dynamic>? route,
+    required Widget? userLeading,
+    required bool automaticallyImplyLeading,
+    required bool automaticallyImplyTitle,
+    required String? previousPageTitle,
+    required Widget? userMiddle,
+    required Widget? userTrailing,
+    required Widget? userLargeTitle,
+    required EdgeInsetsDirectional? padding,
+    required bool large,
   }) : leading = createLeading(
          leadingKey: keys.leadingKey,
          userLeading: userLeading,
@@ -1057,29 +1055,29 @@ class _NavigationBarStaticComponents {
          large: large,
        );
 
-  static Widget _derivedTitle({
-    bool automaticallyImplyTitle,
-    ModalRoute<dynamic> currentRoute,
+  static Widget? _derivedTitle({
+    required bool automaticallyImplyTitle,
+    ModalRoute<dynamic>? currentRoute,
   }) {
     // Auto use the CupertinoPageRoute's title if middle not provided.
     if (automaticallyImplyTitle &&
         currentRoute is CupertinoRouteTransitionMixin &&
         currentRoute.title != null) {
-      return Text(currentRoute.title);
+      return Text(currentRoute.title!);
     }
 
     return null;
   }
 
-  final KeyedSubtree leading;
-  static KeyedSubtree createLeading({
-    @required GlobalKey leadingKey,
-    @required Widget userLeading,
-    @required ModalRoute<dynamic> route,
-    @required bool automaticallyImplyLeading,
-    @required EdgeInsetsDirectional padding,
+  final KeyedSubtree? leading;
+  static KeyedSubtree? createLeading({
+    required GlobalKey leadingKey,
+    required Widget? userLeading,
+    required ModalRoute<dynamic>? route,
+    required bool automaticallyImplyLeading,
+    required EdgeInsetsDirectional? padding,
   }) {
-    Widget leadingContent;
+    Widget? leadingContent;
 
     if (userLeading != null) {
       leadingContent = userLeading;
@@ -1092,7 +1090,7 @@ class _NavigationBarStaticComponents {
       leadingContent = CupertinoButton(
         child: const Text('Close'),
         padding: EdgeInsets.zero,
-        onPressed: () { route.navigator.maybePop(); },
+        onPressed: () { route.navigator!.maybePop(); },
       );
     }
 
@@ -1116,12 +1114,12 @@ class _NavigationBarStaticComponents {
     );
   }
 
-  final KeyedSubtree backChevron;
-  static KeyedSubtree createBackChevron({
-    @required GlobalKey backChevronKey,
-    @required Widget userLeading,
-    @required ModalRoute<dynamic> route,
-    @required bool automaticallyImplyLeading,
+  final KeyedSubtree? backChevron;
+  static KeyedSubtree? createBackChevron({
+    required GlobalKey backChevronKey,
+    required Widget? userLeading,
+    required ModalRoute<dynamic>? route,
+    required bool automaticallyImplyLeading,
   }) {
     if (
       userLeading != null ||
@@ -1138,13 +1136,13 @@ class _NavigationBarStaticComponents {
 
   /// This widget is not decorated with a font since the font style could
   /// animate during transitions.
-  final KeyedSubtree backLabel;
-  static KeyedSubtree createBackLabel({
-    @required GlobalKey backLabelKey,
-    @required Widget userLeading,
-    @required ModalRoute<dynamic> route,
-    @required bool automaticallyImplyLeading,
-    @required String previousPageTitle,
+  final KeyedSubtree? backLabel;
+  static KeyedSubtree? createBackLabel({
+    required GlobalKey backLabelKey,
+    required Widget? userLeading,
+    required ModalRoute<dynamic>? route,
+    required bool automaticallyImplyLeading,
+    required String? previousPageTitle,
   }) {
     if (
       userLeading != null ||
@@ -1167,16 +1165,16 @@ class _NavigationBarStaticComponents {
 
   /// This widget is not decorated with a font since the font style could
   /// animate during transitions.
-  final KeyedSubtree middle;
-  static KeyedSubtree createMiddle({
-    @required GlobalKey middleKey,
-    @required Widget userMiddle,
-    @required Widget userLargeTitle,
-    @required bool large,
-    @required bool automaticallyImplyTitle,
-    @required ModalRoute<dynamic> route,
+  final KeyedSubtree? middle;
+  static KeyedSubtree? createMiddle({
+    required GlobalKey middleKey,
+    required Widget? userMiddle,
+    required Widget? userLargeTitle,
+    required bool large,
+    required bool automaticallyImplyTitle,
+    required ModalRoute<dynamic>? route,
   }) {
-    Widget middleContent = userMiddle;
+    Widget? middleContent = userMiddle;
 
     if (large) {
       middleContent ??= userLargeTitle;
@@ -1197,11 +1195,11 @@ class _NavigationBarStaticComponents {
     );
   }
 
-  final KeyedSubtree trailing;
-  static KeyedSubtree createTrailing({
-    @required GlobalKey trailingKey,
-    @required Widget userTrailing,
-    @required EdgeInsetsDirectional padding,
+  final KeyedSubtree? trailing;
+  static KeyedSubtree? createTrailing({
+    required GlobalKey trailingKey,
+    required Widget? userTrailing,
+    required EdgeInsetsDirectional? padding,
   }) {
     if (userTrailing == null) {
       return null;
@@ -1225,19 +1223,19 @@ class _NavigationBarStaticComponents {
 
   /// This widget is not decorated with a font since the font style could
   /// animate during transitions.
-  final KeyedSubtree largeTitle;
-  static KeyedSubtree createLargeTitle({
-    @required GlobalKey largeTitleKey,
-    @required Widget userLargeTitle,
-    @required bool large,
-    @required bool automaticImplyTitle,
-    @required ModalRoute<dynamic> route,
+  final KeyedSubtree? largeTitle;
+  static KeyedSubtree? createLargeTitle({
+    required GlobalKey largeTitleKey,
+    required Widget? userLargeTitle,
+    required bool large,
+    required bool automaticImplyTitle,
+    required ModalRoute<dynamic>? route,
   }) {
     if (!large) {
       return null;
     }
 
-    final Widget largeTitleContent = userLargeTitle ?? _derivedTitle(
+    final Widget? largeTitleContent = userLargeTitle ?? _derivedTitle(
       automaticallyImplyTitle: automaticImplyTitle,
       currentRoute: route,
     );
@@ -1249,7 +1247,7 @@ class _NavigationBarStaticComponents {
 
     return KeyedSubtree(
       key: largeTitleKey,
-      child: largeTitleContent,
+      child: largeTitleContent!,
     );
   }
 }
@@ -1273,7 +1271,7 @@ class CupertinoNavigationBarBackButton extends StatelessWidget {
   ///
   /// The [color] parameter must not be null.
   const CupertinoNavigationBarBackButton({
-    Key key,
+    Key? key,
     this.color,
     this.previousPageTitle,
     this.onPressed,
@@ -1295,12 +1293,12 @@ class CupertinoNavigationBarBackButton extends StatelessWidget {
   /// Can be used to override the color of the back button chevron and label.
   ///
   /// Defaults to [CupertinoTheme]'s `primaryColor` if null.
-  final Color color;
+  final Color? color;
 
   /// An override for showing the previous route's title. If null, it will be
   /// automatically derived from [CupertinoPageRoute.title] if the current and
   /// previous routes are both [CupertinoPageRoute]s.
-  final String previousPageTitle;
+  final String? previousPageTitle;
 
   /// An override callback to perform instead of the default behavior which is
   /// to pop the [Navigator].
@@ -1310,15 +1308,15 @@ class CupertinoNavigationBarBackButton extends StatelessWidget {
   /// situations.
   ///
   /// Defaults to null.
-  final VoidCallback onPressed;
+  final VoidCallback? onPressed;
 
-  final Widget _backChevron;
+  final Widget? _backChevron;
 
-  final Widget _backLabel;
+  final Widget? _backLabel;
 
   @override
   Widget build(BuildContext context) {
-    final ModalRoute<dynamic> currentRoute = ModalRoute.of(context);
+    final ModalRoute<dynamic>? currentRoute = ModalRoute.of(context);
     if (onPressed == null) {
       assert(
         currentRoute?.canPop == true,
@@ -1362,7 +1360,7 @@ class CupertinoNavigationBarBackButton extends StatelessWidget {
       padding: EdgeInsets.zero,
       onPressed: () {
         if (onPressed != null) {
-          onPressed();
+          onPressed!();
         } else {
           Navigator.maybePop(context);
         }
@@ -1373,12 +1371,12 @@ class CupertinoNavigationBarBackButton extends StatelessWidget {
 
 
 class _BackChevron extends StatelessWidget {
-  const _BackChevron({ Key key }) : super(key: key);
+  const _BackChevron({ Key? key }) : super(key: key);
 
   @override
   Widget build(BuildContext context) {
-    final TextDirection textDirection = Directionality.of(context);
-    final TextStyle textStyle = DefaultTextStyle.of(context).style;
+    final TextDirection textDirection = Directionality.of(context)!;
+    final TextStyle textStyle = DefaultTextStyle.of(context).style!;
 
     // Replicate the Icon logic here to get a tightly sized icon and add
     // custom non-square padding.
@@ -1415,17 +1413,17 @@ class _BackChevron extends StatelessWidget {
 /// is true.
 class _BackLabel extends StatelessWidget {
   const _BackLabel({
-    Key key,
-    @required this.specifiedPreviousTitle,
-    @required this.route,
+    Key? key,
+    required this.specifiedPreviousTitle,
+    required this.route,
   }) : super(key: key);
 
-  final String specifiedPreviousTitle;
-  final ModalRoute<dynamic> route;
+  final String? specifiedPreviousTitle;
+  final ModalRoute<dynamic>? route;
 
   // `child` is never passed in into ValueListenableBuilder so it's always
   // null here and unused.
-  Widget _buildPreviousTitleWidget(BuildContext context, String previousTitle, Widget child) {
+  Widget _buildPreviousTitleWidget(BuildContext context, String? previousTitle, Widget? child) {
     if (previousTitle == null) {
       return const SizedBox(height: 0.0, width: 0.0);
     }
@@ -1451,12 +1449,12 @@ class _BackLabel extends StatelessWidget {
   Widget build(BuildContext context) {
     if (specifiedPreviousTitle != null) {
       return _buildPreviousTitleWidget(context, specifiedPreviousTitle, null);
-    } else if (route is CupertinoRouteTransitionMixin<dynamic> && !route.isFirst) {
+    } else if (route is CupertinoRouteTransitionMixin<dynamic> && !route!.isFirst) {
       final CupertinoRouteTransitionMixin<dynamic> cupertinoRoute = route as CupertinoRouteTransitionMixin<dynamic>;
       // There is no timing issue because the previousTitle Listenable changes
       // happen during route modifications before the ValueListenableBuilder
       // is built.
-      return ValueListenableBuilder<String>(
+      return ValueListenableBuilder<String?>(
         valueListenable: cupertinoRoute.previousTitle,
         builder: _buildPreviousTitleWidget,
       );
@@ -1476,32 +1474,32 @@ class _BackLabel extends StatelessWidget {
 /// navigation bar in each route.
 class _TransitionableNavigationBar extends StatelessWidget {
   _TransitionableNavigationBar({
-    @required this.componentsKeys,
-    @required this.backgroundColor,
-    @required this.backButtonTextStyle,
-    @required this.titleTextStyle,
-    @required this.largeTitleTextStyle,
-    @required this.border,
-    @required this.hasUserMiddle,
-    @required this.largeExpanded,
-    @required this.child,
+    required this.componentsKeys,
+    required this.backgroundColor,
+    required this.backButtonTextStyle,
+    required this.titleTextStyle,
+    required this.largeTitleTextStyle,
+    required this.border,
+    required this.hasUserMiddle,
+    required this.largeExpanded,
+    required this.child,
   }) : assert(componentsKeys != null),
        assert(largeExpanded != null),
        assert(!largeExpanded || largeTitleTextStyle != null),
        super(key: componentsKeys.navBarBoxKey);
 
   final _NavigationBarStaticComponentsKeys componentsKeys;
-  final Color backgroundColor;
+  final Color? backgroundColor;
   final TextStyle backButtonTextStyle;
   final TextStyle titleTextStyle;
-  final TextStyle largeTitleTextStyle;
-  final Border border;
+  final TextStyle? largeTitleTextStyle;
+  final Border? border;
   final bool hasUserMiddle;
   final bool largeExpanded;
   final Widget child;
 
   RenderBox get renderBox {
-    final RenderBox box = componentsKeys.navBarBoxKey.currentContext.findRenderObject() as RenderBox;
+    final RenderBox box = componentsKeys.navBarBoxKey.currentContext!.findRenderObject() as RenderBox;
     assert(
       box.attached,
       '_TransitionableNavigationBar.renderBox should be called when building '
@@ -1514,7 +1512,7 @@ class _TransitionableNavigationBar extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     assert(() {
-      bool inHero;
+      bool inHero = false;
       context.visitAncestorElements((Element ancestor) {
         if (ancestor is ComponentElement) {
           assert(
@@ -1528,11 +1526,10 @@ class _TransitionableNavigationBar extends StatelessWidget {
             inHero = true;
           }
         }
-        inHero ??= false;
         return true;
       });
       assert(
-        inHero == true,
+        inHero,
         '_TransitionableNavigationBar should only be added as the immediate '
         'child of Hero widgets.',
       );
@@ -1559,9 +1556,9 @@ class _TransitionableNavigationBar extends StatelessWidget {
 /// results.
 class _NavigationBarTransition extends StatelessWidget {
   _NavigationBarTransition({
-    @required this.animation,
-    @required this.topNavBar,
-    @required this.bottomNavBar,
+    required this.animation,
+    required this.topNavBar,
+    required this.bottomNavBar,
   }) : heightTween = Tween<double>(
          begin: bottomNavBar.renderBox.size.height,
          end: topNavBar.renderBox.size.height,
@@ -1589,7 +1586,7 @@ class _NavigationBarTransition extends StatelessWidget {
       animation: animation,
       bottomNavBar: bottomNavBar,
       topNavBar: topNavBar,
-      directionality: Directionality.of(context),
+      directionality: Directionality.of(context)!,
     );
 
     final List<Widget> children = <Widget>[
@@ -1597,11 +1594,11 @@ class _NavigationBarTransition extends StatelessWidget {
       // moving components without any components inside it itself.
       AnimatedBuilder(
         animation: animation,
-        builder: (BuildContext context, Widget child) {
+        builder: (BuildContext context, Widget? child) {
           return _wrapWithBackground(
             // Don't update the system status bar color mid-flight.
             updateSystemUiOverlay: false,
-            backgroundColor: backgroundTween.evaluate(animation),
+            backgroundColor: backgroundTween.evaluate(animation)!,
             border: borderTween.evaluate(animation),
             child: SizedBox(
               height: heightTween.evaluate(animation),
@@ -1611,29 +1608,28 @@ class _NavigationBarTransition extends StatelessWidget {
         },
       ),
       // Draw all the components on top of the empty bar box.
-      componentsTransition.bottomBackChevron,
-      componentsTransition.bottomBackLabel,
-      componentsTransition.bottomLeading,
-      componentsTransition.bottomMiddle,
-      componentsTransition.bottomLargeTitle,
-      componentsTransition.bottomTrailing,
+      if (componentsTransition.bottomBackChevron != null) componentsTransition.bottomBackChevron!,
+      if (componentsTransition.bottomBackLabel != null) componentsTransition.bottomBackLabel!,
+      if (componentsTransition.bottomLeading != null) componentsTransition.bottomLeading!,
+      if (componentsTransition.bottomMiddle != null) componentsTransition.bottomMiddle!,
+      if (componentsTransition.bottomLargeTitle != null) componentsTransition.bottomLargeTitle!,
+      if (componentsTransition.bottomTrailing != null) componentsTransition.bottomTrailing!,
       // Draw top components on top of the bottom components.
-      componentsTransition.topLeading,
-      componentsTransition.topBackChevron,
-      componentsTransition.topBackLabel,
-      componentsTransition.topMiddle,
-      componentsTransition.topLargeTitle,
-      componentsTransition.topTrailing,
+      if (componentsTransition.topLeading != null) componentsTransition.topLeading!,
+      if (componentsTransition.topBackChevron != null) componentsTransition.topBackChevron!,
+      if (componentsTransition.topBackLabel != null) componentsTransition.topBackLabel!,
+      if (componentsTransition.topMiddle != null) componentsTransition.topMiddle!,
+      if (componentsTransition.topLargeTitle != null) componentsTransition.topLargeTitle!,
+      if (componentsTransition.topTrailing != null) componentsTransition.topTrailing!,
     ];
 
-    children.removeWhere((Widget child) => child == null);
 
     // The actual outer box is big enough to contain both the bottom and top
     // navigation bars. It's not a direct Rect lerp because some components
     // can actually be outside the linearly lerp'ed Rect in the middle of
     // the animation, such as the topLargeTitle.
     return SizedBox(
-      height: math.max(heightTween.begin, heightTween.end) + MediaQuery.of(context).padding.top,
+      height: math.max(heightTween.begin!, heightTween.end!) + MediaQuery.of(context)!.padding.top,
       width: double.infinity,
       child: Stack(
         children: children,
@@ -1666,10 +1662,10 @@ class _NavigationBarTransition extends StatelessWidget {
 @immutable
 class _NavigationBarComponentsTransition {
   _NavigationBarComponentsTransition({
-    @required this.animation,
-    @required _TransitionableNavigationBar bottomNavBar,
-    @required _TransitionableNavigationBar topNavBar,
-    @required TextDirection directionality,
+    required this.animation,
+    required _TransitionableNavigationBar bottomNavBar,
+    required _TransitionableNavigationBar topNavBar,
+    required TextDirection directionality,
   }) : bottomComponents = bottomNavBar.componentsKeys,
        topComponents = topNavBar.componentsKeys,
        bottomNavBarBox = bottomNavBar.renderBox,
@@ -1712,8 +1708,8 @@ class _NavigationBarComponentsTransition {
   final TextStyle topBackButtonTextStyle;
   final TextStyle bottomTitleTextStyle;
   final TextStyle topTitleTextStyle;
-  final TextStyle bottomLargeTitleTextStyle;
-  final TextStyle topLargeTitleTextStyle;
+  final TextStyle? bottomLargeTitleTextStyle;
+  final TextStyle? topLargeTitleTextStyle;
 
   final bool bottomHasUserMiddle;
   final bool topHasUserMiddle;
@@ -1731,9 +1727,9 @@ class _NavigationBarComponentsTransition {
   // translate it into a RelativeBox in the transition navigation bar box.
   RelativeRect positionInTransitionBox(
     GlobalKey key, {
-    @required RenderBox from,
+    required RenderBox from,
   }) {
-    final RenderBox componentBox = key.currentContext.findRenderObject() as RenderBox;
+    final RenderBox componentBox = key.currentContext!.findRenderObject() as RenderBox;
     assert(componentBox.attached);
 
     return RelativeRect.fromRect(
@@ -1753,15 +1749,15 @@ class _NavigationBarComponentsTransition {
   // BoxConstraints of the 'from' widget so that animating font sizes etc don't
   // produce rounding error artifacts with a linearly resizing rect.
   RelativeRectTween slideFromLeadingEdge({
-    @required GlobalKey fromKey,
-    @required RenderBox fromNavBarBox,
-    @required GlobalKey toKey,
-    @required RenderBox toNavBarBox,
+    required GlobalKey fromKey,
+    required RenderBox fromNavBarBox,
+    required GlobalKey toKey,
+    required RenderBox toNavBarBox,
   }) {
     final RelativeRect fromRect = positionInTransitionBox(fromKey, from: fromNavBarBox);
 
-    final RenderBox fromBox = fromKey.currentContext.findRenderObject() as RenderBox;
-    final RenderBox toBox = toKey.currentContext.findRenderObject() as RenderBox;
+    final RenderBox fromBox = fromKey.currentContext!.findRenderObject() as RenderBox;
+    final RenderBox toBox = toKey.currentContext!.findRenderObject() as RenderBox;
 
     // We move a box with the size of the 'from' render object such that its
     // upper left corner is at the upper left corner of the 'to' render object.
@@ -1799,8 +1795,8 @@ class _NavigationBarComponentsTransition {
     ));
   }
 
-  Widget get bottomLeading {
-    final KeyedSubtree bottomLeading = bottomComponents.leadingKey.currentWidget as KeyedSubtree;
+  Widget? get bottomLeading {
+    final KeyedSubtree? bottomLeading = bottomComponents.leadingKey.currentWidget as KeyedSubtree?;
 
     if (bottomLeading == null) {
       return null;
@@ -1815,8 +1811,8 @@ class _NavigationBarComponentsTransition {
     );
   }
 
-  Widget get bottomBackChevron {
-    final KeyedSubtree bottomBackChevron = bottomComponents.backChevronKey.currentWidget as KeyedSubtree;
+  Widget? get bottomBackChevron {
+    final KeyedSubtree? bottomBackChevron = bottomComponents.backChevronKey.currentWidget as KeyedSubtree?;
 
     if (bottomBackChevron == null) {
       return null;
@@ -1834,8 +1830,8 @@ class _NavigationBarComponentsTransition {
     );
   }
 
-  Widget get bottomBackLabel {
-    final KeyedSubtree bottomBackLabel = bottomComponents.backLabelKey.currentWidget as KeyedSubtree;
+  Widget? get bottomBackLabel {
+    final KeyedSubtree? bottomBackLabel = bottomComponents.backLabelKey.currentWidget as KeyedSubtree?;
 
     if (bottomBackLabel == null) {
       return null;
@@ -1866,10 +1862,10 @@ class _NavigationBarComponentsTransition {
     );
   }
 
-  Widget get bottomMiddle {
-    final KeyedSubtree bottomMiddle = bottomComponents.middleKey.currentWidget as KeyedSubtree;
-    final KeyedSubtree topBackLabel = topComponents.backLabelKey.currentWidget as KeyedSubtree;
-    final KeyedSubtree topLeading = topComponents.leadingKey.currentWidget as KeyedSubtree;
+  Widget? get bottomMiddle {
+    final KeyedSubtree? bottomMiddle = bottomComponents.middleKey.currentWidget as KeyedSubtree?;
+    final KeyedSubtree? topBackLabel = topComponents.backLabelKey.currentWidget as KeyedSubtree?;
+    final KeyedSubtree? topLeading = topComponents.leadingKey.currentWidget as KeyedSubtree?;
 
     // The middle component is non-null when the nav bar is a large title
     // nav bar but would be invisible when expanded, therefore don't show it here.
@@ -1925,10 +1921,10 @@ class _NavigationBarComponentsTransition {
     return null;
   }
 
-  Widget get bottomLargeTitle {
-    final KeyedSubtree bottomLargeTitle = bottomComponents.largeTitleKey.currentWidget as KeyedSubtree;
-    final KeyedSubtree topBackLabel = topComponents.backLabelKey.currentWidget as KeyedSubtree;
-    final KeyedSubtree topLeading = topComponents.leadingKey.currentWidget as KeyedSubtree;
+  Widget? get bottomLargeTitle {
+    final KeyedSubtree? bottomLargeTitle = bottomComponents.largeTitleKey.currentWidget as KeyedSubtree?;
+    final KeyedSubtree? topBackLabel = topComponents.backLabelKey.currentWidget as KeyedSubtree?;
+    final KeyedSubtree? topLeading = topComponents.leadingKey.currentWidget as KeyedSubtree?;
 
     if (bottomLargeTitle == null || !bottomLargeExpanded) {
       return null;
@@ -1996,8 +1992,8 @@ class _NavigationBarComponentsTransition {
     return null;
   }
 
-  Widget get bottomTrailing {
-    final KeyedSubtree bottomTrailing = bottomComponents.trailingKey.currentWidget as KeyedSubtree;
+  Widget? get bottomTrailing {
+    final KeyedSubtree? bottomTrailing = bottomComponents.trailingKey.currentWidget as KeyedSubtree?;
 
     if (bottomTrailing == null) {
       return null;
@@ -2012,8 +2008,8 @@ class _NavigationBarComponentsTransition {
     );
   }
 
-  Widget get topLeading {
-    final KeyedSubtree topLeading = topComponents.leadingKey.currentWidget as KeyedSubtree;
+  Widget? get topLeading {
+    final KeyedSubtree? topLeading = topComponents.leadingKey.currentWidget as KeyedSubtree?;
 
     if (topLeading == null) {
       return null;
@@ -2028,9 +2024,9 @@ class _NavigationBarComponentsTransition {
     );
   }
 
-  Widget get topBackChevron {
-    final KeyedSubtree topBackChevron = topComponents.backChevronKey.currentWidget as KeyedSubtree;
-    final KeyedSubtree bottomBackChevron = bottomComponents.backChevronKey.currentWidget as KeyedSubtree;
+  Widget? get topBackChevron {
+    final KeyedSubtree? topBackChevron = topComponents.backChevronKey.currentWidget as KeyedSubtree?;
+    final KeyedSubtree? bottomBackChevron = bottomComponents.backChevronKey.currentWidget as KeyedSubtree?;
 
     if (topBackChevron == null) {
       return null;
@@ -2042,7 +2038,7 @@ class _NavigationBarComponentsTransition {
     // If it's the first page with a back chevron, shift in slightly from the
     // right.
     if (bottomBackChevron == null) {
-      final RenderBox topBackChevronBox = topComponents.backChevronKey.currentContext.findRenderObject() as RenderBox;
+      final RenderBox topBackChevronBox = topComponents.backChevronKey.currentContext!.findRenderObject() as RenderBox;
       from = to.shift(
         Offset(
           forwardDirection * topBackChevronBox.size.width * 2.0,
@@ -2068,19 +2064,19 @@ class _NavigationBarComponentsTransition {
     );
   }
 
-  Widget get topBackLabel {
-    final KeyedSubtree bottomMiddle = bottomComponents.middleKey.currentWidget as KeyedSubtree;
-    final KeyedSubtree bottomLargeTitle = bottomComponents.largeTitleKey.currentWidget as KeyedSubtree;
-    final KeyedSubtree topBackLabel = topComponents.backLabelKey.currentWidget as KeyedSubtree;
+  Widget? get topBackLabel {
+    final KeyedSubtree? bottomMiddle = bottomComponents.middleKey.currentWidget as KeyedSubtree?;
+    final KeyedSubtree? bottomLargeTitle = bottomComponents.largeTitleKey.currentWidget as KeyedSubtree?;
+    final KeyedSubtree? topBackLabel = topComponents.backLabelKey.currentWidget as KeyedSubtree?;
 
     if (topBackLabel == null) {
       return null;
     }
 
-    final RenderAnimatedOpacity topBackLabelOpacity =
+    final RenderAnimatedOpacity? topBackLabelOpacity =
         topComponents.backLabelKey.currentContext?.findAncestorRenderObjectOfType<RenderAnimatedOpacity>();
 
-    Animation<double> midClickOpacity;
+    Animation<double>? midClickOpacity;
     if (topBackLabelOpacity != null && topBackLabelOpacity.opacity.value < 1.0) {
       midClickOpacity = animation.drive(Tween<double>(
         begin: 0.0,
@@ -2144,8 +2140,8 @@ class _NavigationBarComponentsTransition {
     return null;
   }
 
-  Widget get topMiddle {
-    final KeyedSubtree topMiddle = topComponents.middleKey.currentWidget as KeyedSubtree;
+  Widget? get topMiddle {
+    final KeyedSubtree? topMiddle = topComponents.middleKey.currentWidget as KeyedSubtree?;
 
     if (topMiddle == null) {
       return null;
@@ -2182,8 +2178,8 @@ class _NavigationBarComponentsTransition {
     );
   }
 
-  Widget get topTrailing {
-    final KeyedSubtree topTrailing = topComponents.trailingKey.currentWidget as KeyedSubtree;
+  Widget? get topTrailing {
+    final KeyedSubtree? topTrailing = topComponents.trailingKey.currentWidget as KeyedSubtree?;
 
     if (topTrailing == null) {
       return null;
@@ -2198,8 +2194,8 @@ class _NavigationBarComponentsTransition {
     );
   }
 
-  Widget get topLargeTitle {
-    final KeyedSubtree topLargeTitle = topComponents.largeTitleKey.currentWidget as KeyedSubtree;
+  Widget? get topLargeTitle {
+    final KeyedSubtree? topLargeTitle = topComponents.largeTitleKey.currentWidget as KeyedSubtree?;
 
     if (topLargeTitle == null || !topLargeExpanded) {
       return null;
@@ -2235,9 +2231,9 @@ class _NavigationBarComponentsTransition {
 
 /// Navigation bars' hero rect tween that will move between the static bars
 /// but keep a constant size that's the bigger of both navigation bars.
-CreateRectTween _linearTranslateWithLargestRectSizeTween = (Rect begin, Rect end) {
+CreateRectTween _linearTranslateWithLargestRectSizeTween = (Rect? begin, Rect? end) {
   final Size largestSize = Size(
-    math.max(begin.size.width, end.size.width),
+    math.max(begin!.size.width, end!.size.width),
     math.max(begin.size.height, end.size.height),
   );
   return RectTween(
@@ -2300,11 +2296,11 @@ final HeroFlightShuttleBuilder _navBarHeroFlightShuttleBuilder = (
   assert(toNavBar.componentsKeys != null);
 
   assert(
-    fromNavBar.componentsKeys.navBarBoxKey.currentContext.owner != null,
+    fromNavBar.componentsKeys.navBarBoxKey.currentContext!.owner != null,
     'The from nav bar to Hero must have been mounted in the previous frame',
   );
   assert(
-    toNavBar.componentsKeys.navBarBoxKey.currentContext.owner != null,
+    toNavBar.componentsKeys.navBarBoxKey.currentContext!.owner != null,
     'The to nav bar to Hero must have been mounted in the previous frame',
   );
 
@@ -2315,7 +2311,6 @@ final HeroFlightShuttleBuilder _navBarHeroFlightShuttleBuilder = (
         bottomNavBar: fromNavBar,
         topNavBar: toNavBar,
       );
-      break;
     case HeroFlightDirection.pop:
       return _NavigationBarTransition(
         animation: animation,
@@ -2323,5 +2318,4 @@ final HeroFlightShuttleBuilder _navBarHeroFlightShuttleBuilder = (
         topNavBar: fromNavBar,
       );
   }
-  return null;
 };

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

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter/widgets.dart';
 
 import 'colors.dart';
@@ -22,11 +20,11 @@ import 'theme.dart';
 class CupertinoPageScaffold extends StatefulWidget {
   /// Creates a layout for pages with a navigation bar at the top.
   const CupertinoPageScaffold({
-    Key key,
+    Key? key,
     this.navigationBar,
     this.backgroundColor,
     this.resizeToAvoidBottomInset = true,
-    @required this.child,
+    required this.child,
   }) : assert(child != null),
        assert(resizeToAvoidBottomInset != null),
        super(key: key);
@@ -48,7 +46,7 @@ class CupertinoPageScaffold extends StatefulWidget {
   /// in many ways, such as querying [MediaQuery.textScaleFactorOf] against
   /// [CupertinoApp]'s [BuildContext].
   // TODO(xster): document its page transition animation when ready
-  final ObstructingPreferredSizeWidget navigationBar;
+  final ObstructingPreferredSizeWidget? navigationBar;
 
   /// Widget to show in the main content area.
   ///
@@ -61,7 +59,7 @@ class CupertinoPageScaffold extends StatefulWidget {
   /// The color of the widget that underlies the entire scaffold.
   ///
   /// By default uses [CupertinoTheme]'s `scaffoldBackgroundColor` when null.
-  final Color backgroundColor;
+  final Color? backgroundColor;
 
   /// Whether the [child] should size itself to avoid the window's bottom inset.
   ///
@@ -95,12 +93,12 @@ class _CupertinoPageScaffoldState extends State<CupertinoPageScaffold> {
   Widget build(BuildContext context) {
     Widget paddedContent = widget.child;
 
-    final MediaQueryData existingMediaQuery = MediaQuery.of(context);
+    final MediaQueryData existingMediaQuery = MediaQuery.of(context)!;
     if (widget.navigationBar != null) {
       // TODO(xster): Use real size after partial layout instead of preferred size.
       // https://github.com/flutter/flutter/issues/12912
       final double topPadding =
-          widget.navigationBar.preferredSize.height + existingMediaQuery.padding.top;
+          widget.navigationBar!.preferredSize.height + existingMediaQuery.padding.top;
 
       // Propagate bottom padding and include viewInsets if appropriate
       final double bottomPadding = widget.resizeToAvoidBottomInset
@@ -113,7 +111,7 @@ class _CupertinoPageScaffoldState extends State<CupertinoPageScaffold> {
           ? existingMediaQuery.viewInsets.copyWith(bottom: 0.0)
           : existingMediaQuery.viewInsets;
 
-      final bool fullObstruction = widget.navigationBar.shouldFullyObstruct(context);
+      final bool fullObstruction = widget.navigationBar!.shouldFullyObstruct(context);
 
       // If navigation bar is opaquely obstructing, directly shift the main content
       // down. If translucent, let main content draw behind navigation bar but hint the
@@ -176,7 +174,7 @@ class _CupertinoPageScaffoldState extends State<CupertinoPageScaffold> {
               right: 0.0,
               child: MediaQuery(
                 data: existingMediaQuery.copyWith(textScaleFactor: 1),
-                child: widget.navigationBar,
+                child: widget.navigationBar!,
               ),
             ),
           // Add a touch handler the size of the status bar on top of all contents

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

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:collection';
 import 'dart:math' as math;
 
@@ -66,7 +64,7 @@ const Duration _kFadeDuration = Duration(milliseconds: 165);
 ///  * [CupertinoSegmentedControl], a segmented control widget in the style used
 ///    up until iOS 13.
 ///  * <https://developer.apple.com/design/human-interface-guidelines/ios/controls/segmented-controls/>
-class CupertinoSegmentedControl<T> extends StatefulWidget {
+class CupertinoSegmentedControl<T extends Object> extends StatefulWidget {
   /// Creates an iOS-style segmented control bar.
   ///
   /// The [children] and [onValueChanged] arguments must not be null. The
@@ -83,9 +81,9 @@ class CupertinoSegmentedControl<T> extends StatefulWidget {
   /// appear as selected. The [groupValue] must be either null or one of the keys
   /// in the [children] map.
   CupertinoSegmentedControl({
-    Key key,
-    @required this.children,
-    @required this.onValueChanged,
+    Key? key,
+    required this.children,
+    required this.onValueChanged,
     this.groupValue,
     this.unselectedColor,
     this.selectedColor,
@@ -112,7 +110,7 @@ class CupertinoSegmentedControl<T> extends StatefulWidget {
   ///
   /// This must be one of the keys in the [Map] of [children].
   /// If this attribute is null, no widget will be initially selected.
-  final T groupValue;
+  final T? groupValue;
 
   /// The callback that is called when a new option is tapped.
   ///
@@ -165,49 +163,49 @@ class CupertinoSegmentedControl<T> extends StatefulWidget {
   /// text color of the selected widget.
   ///
   /// Defaults to [CupertinoTheme]'s `primaryContrastingColor` if null.
-  final Color unselectedColor;
+  final Color? unselectedColor;
 
   /// The color used to fill the background of the selected widget and as the text
   /// color of unselected widgets.
   ///
   /// Defaults to [CupertinoTheme]'s `primaryColor` if null.
-  final Color selectedColor;
+  final Color? selectedColor;
 
   /// The color used as the border around each widget.
   ///
   /// Defaults to [CupertinoTheme]'s `primaryColor` if null.
-  final Color borderColor;
+  final Color? borderColor;
 
   /// The color used to fill the background of the widget the user is
   /// temporarily interacting with through a long press or drag.
   ///
   /// Defaults to the selectedColor at 20% opacity if null.
-  final Color pressedColor;
+  final Color? pressedColor;
 
   /// The CupertinoSegmentedControl will be placed inside this padding.
   ///
   /// Defaults to EdgeInsets.symmetric(horizontal: 16.0)
-  final EdgeInsetsGeometry padding;
+  final EdgeInsetsGeometry? padding;
 
   @override
   _SegmentedControlState<T> createState() => _SegmentedControlState<T>();
 }
 
-class _SegmentedControlState<T> extends State<CupertinoSegmentedControl<T>>
+class _SegmentedControlState<T extends Object> extends State<CupertinoSegmentedControl<T>>
     with TickerProviderStateMixin<CupertinoSegmentedControl<T>> {
-  T _pressedKey;
+  T? _pressedKey;
 
   final List<AnimationController> _selectionControllers = <AnimationController>[];
   final List<ColorTween> _childTweens = <ColorTween>[];
 
-  ColorTween _forwardBackgroundColorTween;
-  ColorTween _reverseBackgroundColorTween;
-  ColorTween _textColorTween;
+  late ColorTween _forwardBackgroundColorTween;
+  late ColorTween _reverseBackgroundColorTween;
+  late ColorTween _textColorTween;
 
-  Color _selectedColor;
-  Color _unselectedColor;
-  Color _borderColor;
-  Color _pressedColor;
+  Color? _selectedColor;
+  Color? _unselectedColor;
+  Color? _borderColor;
+  Color? _pressedColor;
 
   AnimationController createAnimationController() {
     return AnimationController(
@@ -343,7 +341,7 @@ class _SegmentedControlState<T> extends State<CupertinoSegmentedControl<T>>
     _pressedKey = null;
   }
 
-  Color getTextColor(int index, T currentKey) {
+  Color? getTextColor(int index, T currentKey) {
     if (_selectionControllers[index].isAnimating)
       return _textColorTween.evaluate(_selectionControllers[index]);
     if (widget.groupValue == currentKey)
@@ -351,7 +349,7 @@ class _SegmentedControlState<T> extends State<CupertinoSegmentedControl<T>>
     return _selectedColor;
   }
 
-  Color getBackgroundColor(int index, T currentKey) {
+  Color? getBackgroundColor(int index, T currentKey) {
     if (_selectionControllers[index].isAnimating)
       return _childTweens[index].evaluate(_selectionControllers[index]);
     if (widget.groupValue == currentKey)
@@ -366,13 +364,13 @@ class _SegmentedControlState<T> extends State<CupertinoSegmentedControl<T>>
     final List<Widget> _gestureChildren = <Widget>[];
     final List<Color> _backgroundColors = <Color>[];
     int index = 0;
-    int selectedIndex;
-    int pressedIndex;
+    int? selectedIndex;
+    int? pressedIndex;
     for (final T currentKey in widget.children.keys) {
       selectedIndex = (widget.groupValue == currentKey) ? index : selectedIndex;
       pressedIndex = (_pressedKey == currentKey) ? index : pressedIndex;
 
-      final TextStyle textStyle = DefaultTextStyle.of(context).style.copyWith(
+      final TextStyle textStyle = DefaultTextStyle.of(context).style!.copyWith(
         color: getTextColor(index, currentKey),
       );
       final IconThemeData iconTheme = IconThemeData(
@@ -406,7 +404,7 @@ class _SegmentedControlState<T> extends State<CupertinoSegmentedControl<T>>
         ),
       );
 
-      _backgroundColors.add(getBackgroundColor(index, currentKey));
+      _backgroundColors.add(getBackgroundColor(index, currentKey)!);
       _gestureChildren.add(child);
       index += 1;
     }
@@ -416,7 +414,7 @@ class _SegmentedControlState<T> extends State<CupertinoSegmentedControl<T>>
       selectedIndex: selectedIndex,
       pressedIndex: pressedIndex,
       backgroundColors: _backgroundColors,
-      borderColor: _borderColor,
+      borderColor: _borderColor!,
     );
 
     return Padding(
@@ -431,26 +429,26 @@ class _SegmentedControlState<T> extends State<CupertinoSegmentedControl<T>>
 
 class _SegmentedControlRenderWidget<T> extends MultiChildRenderObjectWidget {
   _SegmentedControlRenderWidget({
-    Key key,
+    Key? key,
     List<Widget> children = const <Widget>[],
-    @required this.selectedIndex,
-    @required this.pressedIndex,
-    @required this.backgroundColors,
-    @required this.borderColor,
+    required this.selectedIndex,
+    required this.pressedIndex,
+    required this.backgroundColors,
+    required this.borderColor,
   }) : super(
           key: key,
           children: children,
         );
 
-  final int selectedIndex;
-  final int pressedIndex;
+  final int? selectedIndex;
+  final int? pressedIndex;
   final List<Color> backgroundColors;
   final Color borderColor;
 
   @override
   RenderObject createRenderObject(BuildContext context) {
     return _RenderSegmentedControl<T>(
-      textDirection: Directionality.of(context),
+      textDirection: Directionality.of(context)!,
       selectedIndex: selectedIndex,
       pressedIndex: pressedIndex,
       backgroundColors: backgroundColors,
@@ -461,7 +459,7 @@ class _SegmentedControlRenderWidget<T> extends MultiChildRenderObjectWidget {
   @override
   void updateRenderObject(BuildContext context, _RenderSegmentedControl<T> renderObject) {
     renderObject
-      ..textDirection = Directionality.of(context)
+      ..textDirection = Directionality.of(context)!
       ..selectedIndex = selectedIndex
       ..pressedIndex = pressedIndex
       ..backgroundColors = backgroundColors
@@ -470,20 +468,20 @@ class _SegmentedControlRenderWidget<T> extends MultiChildRenderObjectWidget {
 }
 
 class _SegmentedControlContainerBoxParentData extends ContainerBoxParentData<RenderBox> {
-  RRect surroundingRect;
+  RRect? surroundingRect;
 }
 
-typedef _NextChild = RenderBox Function(RenderBox child);
+typedef _NextChild = RenderBox? Function(RenderBox child);
 
 class _RenderSegmentedControl<T> extends RenderBox
     with ContainerRenderObjectMixin<RenderBox, ContainerBoxParentData<RenderBox>>,
         RenderBoxContainerDefaultsMixin<RenderBox, ContainerBoxParentData<RenderBox>> {
   _RenderSegmentedControl({
-    @required int selectedIndex,
-    @required int pressedIndex,
-    @required TextDirection textDirection,
-    @required List<Color> backgroundColors,
-    @required Color borderColor,
+    required int? selectedIndex,
+    required int? pressedIndex,
+    required TextDirection textDirection,
+    required List<Color> backgroundColors,
+    required Color borderColor,
   }) : assert(textDirection != null),
        _textDirection = textDirection,
        _selectedIndex = selectedIndex,
@@ -491,9 +489,9 @@ class _RenderSegmentedControl<T> extends RenderBox
        _backgroundColors = backgroundColors,
        _borderColor = borderColor;
 
-  int get selectedIndex => _selectedIndex;
-  int _selectedIndex;
-  set selectedIndex(int value) {
+  int? get selectedIndex => _selectedIndex;
+  int? _selectedIndex;
+  set selectedIndex(int? value) {
     if (_selectedIndex == value) {
       return;
     }
@@ -501,9 +499,9 @@ class _RenderSegmentedControl<T> extends RenderBox
     markNeedsPaint();
   }
 
-  int get pressedIndex => _pressedIndex;
-  int _pressedIndex;
-  set pressedIndex(int value) {
+  int? get pressedIndex => _pressedIndex;
+  int? _pressedIndex;
+  set pressedIndex(int? value) {
     if (_pressedIndex == value) {
       return;
     }
@@ -543,7 +541,7 @@ class _RenderSegmentedControl<T> extends RenderBox
 
   @override
   double computeMinIntrinsicWidth(double height) {
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     double minWidth = 0.0;
     while (child != null) {
       final _SegmentedControlContainerBoxParentData childParentData = child.parentData as _SegmentedControlContainerBoxParentData;
@@ -556,7 +554,7 @@ class _RenderSegmentedControl<T> extends RenderBox
 
   @override
   double computeMaxIntrinsicWidth(double height) {
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     double maxWidth = 0.0;
     while (child != null) {
       final _SegmentedControlContainerBoxParentData childParentData = child.parentData as _SegmentedControlContainerBoxParentData;
@@ -569,7 +567,7 @@ class _RenderSegmentedControl<T> extends RenderBox
 
   @override
   double computeMinIntrinsicHeight(double width) {
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     double minHeight = 0.0;
     while (child != null) {
       final _SegmentedControlContainerBoxParentData childParentData = child.parentData as _SegmentedControlContainerBoxParentData;
@@ -582,7 +580,7 @@ class _RenderSegmentedControl<T> extends RenderBox
 
   @override
   double computeMaxIntrinsicHeight(double width) {
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     double maxHeight = 0.0;
     while (child != null) {
       final _SegmentedControlContainerBoxParentData childParentData = child.parentData as _SegmentedControlContainerBoxParentData;
@@ -594,7 +592,7 @@ class _RenderSegmentedControl<T> extends RenderBox
   }
 
   @override
-  double computeDistanceToActualBaseline(TextBaseline baseline) {
+  double? computeDistanceToActualBaseline(TextBaseline baseline) {
     return defaultComputeDistanceToHighestActualBaseline(baseline);
   }
 
@@ -605,8 +603,8 @@ class _RenderSegmentedControl<T> extends RenderBox
     }
   }
 
-  void _layoutRects(_NextChild nextChild, RenderBox leftChild, RenderBox rightChild) {
-    RenderBox child = leftChild;
+  void _layoutRects(_NextChild nextChild, RenderBox? leftChild, RenderBox? rightChild) {
+    RenderBox? child = leftChild;
     double start = 0.0;
     while (child != null) {
       final _SegmentedControlContainerBoxParentData childParentData = child.parentData as _SegmentedControlContainerBoxParentData;
@@ -640,7 +638,7 @@ class _RenderSegmentedControl<T> extends RenderBox
     }
     childWidth = math.min(childWidth, constraints.maxWidth / childCount);
 
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     while (child != null) {
       final double boxHeight = child.getMaxIntrinsicHeight(childWidth);
       maxHeight = math.max(maxHeight, boxHeight);
@@ -682,7 +680,7 @@ class _RenderSegmentedControl<T> extends RenderBox
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     int index = 0;
     while (child != null) {
       _paintChild(context, offset, child, index);
@@ -697,13 +695,13 @@ class _RenderSegmentedControl<T> extends RenderBox
     final _SegmentedControlContainerBoxParentData childParentData = child.parentData as _SegmentedControlContainerBoxParentData;
 
     context.canvas.drawRRect(
-      childParentData.surroundingRect.shift(offset),
+      childParentData.surroundingRect!.shift(offset),
       Paint()
         ..color = backgroundColors[childIndex]
         ..style = PaintingStyle.fill,
     );
     context.canvas.drawRRect(
-      childParentData.surroundingRect.shift(offset),
+      childParentData.surroundingRect!.shift(offset),
       Paint()
         ..color = borderColor
         ..strokeWidth = 1.0
@@ -714,18 +712,18 @@ class _RenderSegmentedControl<T> extends RenderBox
   }
 
   @override
-  bool hitTestChildren(BoxHitTestResult result, { @required Offset position }) {
+  bool hitTestChildren(BoxHitTestResult result, { required Offset position }) {
     assert(position != null);
-    RenderBox child = lastChild;
+    RenderBox? child = lastChild;
     while (child != null) {
       final _SegmentedControlContainerBoxParentData childParentData = child.parentData as _SegmentedControlContainerBoxParentData;
-      if (childParentData.surroundingRect.contains(position)) {
+      if (childParentData.surroundingRect!.contains(position)) {
         return result.addWithPaintOffset(
           offset: childParentData.offset,
           position: position,
           hitTest: (BoxHitTestResult result, Offset localOffset) {
             assert(localOffset == position - childParentData.offset);
-            return child.hitTest(result, position: localOffset);
+            return child!.hitTest(result, position: localOffset);
           },
         );
       }

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

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:math' as math;
 
 import 'package:flutter/foundation.dart';
@@ -76,10 +74,10 @@ const Duration _kOpacityAnimationDuration = Duration(milliseconds: 470);
 const Duration _kHighlightAnimationDuration = Duration(milliseconds: 200);
 
 class _FontWeightTween extends Tween<FontWeight> {
-  _FontWeightTween({ FontWeight begin, FontWeight end }) : super(begin: begin, end: end);
+  _FontWeightTween({ required FontWeight begin, required FontWeight end }) : super(begin: begin, end: end);
 
   @override
-  FontWeight lerp(double t) => FontWeight.lerp(begin, end, t);
+  FontWeight lerp(double t) => FontWeight.lerp(begin, end, t)!;
 }
 
 /// An iOS 13 style segmented control.
@@ -140,9 +138,9 @@ class CupertinoSlidingSegmentedControl<T> extends StatefulWidget {
   /// appear as selected. The [groupValue] must be either null or one of the keys
   /// in the [children] map.
   CupertinoSlidingSegmentedControl({
-    Key key,
-    @required this.children,
-    @required this.onValueChanged,
+    Key? key,
+    required this.children,
+    required this.onValueChanged,
     this.groupValue,
     this.thumbColor = _kThumbColor,
     this.padding = _kHorizontalItemPadding,
@@ -168,7 +166,7 @@ class CupertinoSlidingSegmentedControl<T> extends StatefulWidget {
   ///
   /// This must be one of the keys in the [Map] of [children].
   /// If this attribute is null, no widget will be initially selected.
-  final T groupValue;
+  final T? groupValue;
 
   /// The callback that is called when a new option is tapped.
   ///
@@ -215,7 +213,7 @@ class CupertinoSlidingSegmentedControl<T> extends StatefulWidget {
   /// }
   /// ```
   /// {@end-tool}
-  final ValueChanged<T> onValueChanged;
+  final ValueChanged<T?> onValueChanged;
 
   /// The color used to paint the rounded rect behind the [children] and the separators.
   ///
@@ -248,11 +246,11 @@ class _SegmentedControlState<T> extends State<CupertinoSlidingSegmentedControl<T
   final Map<T, AnimationController> _pressControllers = <T, AnimationController>{};
   final Tween<double> _pressTween = Tween<double>(begin: 1, end: 0.2);
 
-  List<T> keys;
+  late List<T> keys;
 
-  AnimationController thumbController;
-  AnimationController separatorOpacityController;
-  AnimationController thumbScaleController;
+  late AnimationController thumbController;
+  late AnimationController separatorOpacityController;
+  late AnimationController thumbScaleController;
 
   final TapGestureRecognizer tap = TapGestureRecognizer();
   final HorizontalDragGestureRecognizer drag = HorizontalDragGestureRecognizer();
@@ -320,8 +318,8 @@ class _SegmentedControlState<T> extends State<CupertinoSlidingSegmentedControl<T
     // Update animation controllers.
     for (final T oldKey in oldWidget.children.keys) {
       if (!widget.children.containsKey(oldKey)) {
-        _highlightControllers[oldKey].dispose();
-        _pressControllers[oldKey].dispose();
+        _highlightControllers[oldKey]!.dispose();
+        _pressControllers[oldKey]!.dispose();
 
         _highlightControllers.remove(oldKey);
         _pressControllers.remove(oldKey);
@@ -360,16 +358,16 @@ class _SegmentedControlState<T> extends State<CupertinoSlidingSegmentedControl<T
   }
 
   // Play highlight animation for the child located at _highlightControllers[at].
-  void _animateHighlightController({ T at, bool forward }) {
+  void _animateHighlightController({ T? at, required bool forward }) {
     if (at == null)
       return;
-    final AnimationController controller = _highlightControllers[at];
+    final AnimationController? controller = _highlightControllers[at];
     assert(!forward || controller != null);
     controller?.animateTo(forward ? 1 : 0, duration: _kHighlightAnimationDuration, curve: Curves.ease);
   }
 
-  T _highlighted;
-  set highlighted(T newValue) {
+  T? _highlighted;
+  set highlighted(T? newValue) {
     if (_highlighted == newValue)
       return;
     _animateHighlightController(at: newValue, forward: true);
@@ -377,8 +375,8 @@ class _SegmentedControlState<T> extends State<CupertinoSlidingSegmentedControl<T
     _highlighted = newValue;
   }
 
-  T _pressed;
-  set pressed(T newValue) {
+  T? _pressed;
+  set pressed(T? newValue) {
     if (_pressed == newValue)
       return;
 
@@ -386,7 +384,7 @@ class _SegmentedControlState<T> extends State<CupertinoSlidingSegmentedControl<T
       _pressControllers[_pressed]?.animateTo(0, duration: _kOpacityAnimationDuration, curve: Curves.ease);
     }
     if (newValue != _highlighted && newValue != null) {
-      _pressControllers[newValue].animateTo(1, duration: _kOpacityAnimationDuration, curve: Curves.ease);
+      _pressControllers[newValue]!.animateTo(1, duration: _kOpacityAnimationDuration, curve: Curves.ease);
     }
     _pressed = newValue;
   }
@@ -395,13 +393,13 @@ class _SegmentedControlState<T> extends State<CupertinoSlidingSegmentedControl<T
     widget.onValueChanged(_highlighted);
   }
 
-  T indexToKey(int index) => index == null ? null : keys[index];
+  T? indexToKey(int? index) => index == null ? null : keys[index];
 
   @override
   Widget build(BuildContext context) {
     debugCheckHasDirectionality(context);
 
-    switch (Directionality.of(context)) {
+    switch (Directionality.of(context)!) {
       case TextDirection.ltr:
         keys = widget.children.keys.toList(growable: false);
         break;
@@ -415,11 +413,11 @@ class _SegmentedControlState<T> extends State<CupertinoSlidingSegmentedControl<T
         ..._highlightControllers.values,
         ..._pressControllers.values,
       ]),
-      builder: (BuildContext context, Widget child) {
+      builder: (BuildContext context, Widget? child) {
         final List<Widget> children = <Widget>[];
         for (final T currentKey in keys) {
-          final TextStyle textStyle = DefaultTextStyle.of(context).style.copyWith(
-            fontWeight: _highlightTween.evaluate(_highlightControllers[currentKey]),
+          final TextStyle textStyle = DefaultTextStyle.of(context).style!.copyWith(
+            fontWeight: _highlightTween.evaluate(_highlightControllers[currentKey]!),
           );
 
           final Widget child = DefaultTextStyle(
@@ -430,7 +428,7 @@ class _SegmentedControlState<T> extends State<CupertinoSlidingSegmentedControl<T
               inMutuallyExclusiveGroup: true,
               selected: widget.groupValue == currentKey,
               child: Opacity(
-                opacity: _pressTween.evaluate(_pressControllers[currentKey]),
+                opacity: _pressTween.evaluate(_pressControllers[currentKey]!),
                 // Expand the hitTest area to be as large as the Opacity widget.
                 child: MetaData(
                   behavior: HitTestBehavior.opaque,
@@ -443,7 +441,7 @@ class _SegmentedControlState<T> extends State<CupertinoSlidingSegmentedControl<T
           children.add(child);
         }
 
-        final int selectedIndex = widget.groupValue == null ? null : keys.indexOf(widget.groupValue);
+        final int? selectedIndex = widget.groupValue == null ? null : keys.indexOf(widget.groupValue as T);
 
         final Widget box = _SegmentedControlRenderWidget<T>(
           children: children,
@@ -470,15 +468,15 @@ class _SegmentedControlState<T> extends State<CupertinoSlidingSegmentedControl<T
 
 class _SegmentedControlRenderWidget<T> extends MultiChildRenderObjectWidget {
   _SegmentedControlRenderWidget({
-    Key key,
+    Key? key,
     List<Widget> children = const <Widget>[],
-    @required this.selectedIndex,
-    @required this.thumbColor,
-    @required this.state,
+    required this.selectedIndex,
+    required this.thumbColor,
+    required this.state,
   }) : super(key: key, children: children);
 
-  final int selectedIndex;
-  final Color thumbColor;
+  final int? selectedIndex;
+  final Color? thumbColor;
   final _SegmentedControlState<T> state;
 
   @override
@@ -501,7 +499,7 @@ class _SegmentedControlRenderWidget<T> extends MultiChildRenderObjectWidget {
 class _ChildAnimationManifest {
   _ChildAnimationManifest({
     this.opacity = 1,
-    @required this.separatorOpacity,
+    required this.separatorOpacity,
   }) : assert(separatorOpacity != null),
        assert(opacity != null),
        separatorTween = Tween<double>(begin: separatorOpacity, end: separatorOpacity),
@@ -550,9 +548,9 @@ class _RenderSegmentedControl<T> extends RenderBox
     with ContainerRenderObjectMixin<RenderBox, ContainerBoxParentData<RenderBox>>,
         RenderBoxContainerDefaultsMixin<RenderBox, ContainerBoxParentData<RenderBox>> {
   _RenderSegmentedControl({
-    @required int selectedIndex,
-    @required Color thumbColor,
-    @required this.state,
+    required int? selectedIndex,
+    required Color? thumbColor,
+    required this.state,
   }) : _highlightedIndex = selectedIndex,
        _thumbColor = thumbColor,
        assert(state != null) {
@@ -569,29 +567,29 @@ class _RenderSegmentedControl<T> extends RenderBox
 
   final _SegmentedControlState<T> state;
 
-  Map<RenderBox, _ChildAnimationManifest> _childAnimations = <RenderBox, _ChildAnimationManifest>{};
+  Map<RenderBox, _ChildAnimationManifest>? _childAnimations = <RenderBox, _ChildAnimationManifest>{};
 
   // The current **Unscaled** Thumb Rect.
-  Rect currentThumbRect;
+  Rect? currentThumbRect;
 
-  Tween<Rect> _currentThumbTween;
+  Tween<Rect?>? _currentThumbTween;
 
   Tween<double> _thumbScaleTween = Tween<double>(begin: _kMinThumbScale, end: 1);
   double currentThumbScale = 1;
 
   // The current position of the active drag pointer.
-  Offset _localDragOffset;
+  Offset? _localDragOffset;
   // Whether the current drag gesture started on a selected segment.
-  bool _startedOnSelectedSegment;
+  bool? _startedOnSelectedSegment;
 
   @override
-  void insert(RenderBox child, { RenderBox after }) {
+  void insert(RenderBox child, { RenderBox? after }) {
     super.insert(child, after: after);
     if (_childAnimations == null)
       return;
 
-    assert(_childAnimations[child] == null);
-    _childAnimations[child] = _ChildAnimationManifest(separatorOpacity: 1);
+    assert(_childAnimations![child] == null);
+    _childAnimations![child] = _ChildAnimationManifest(separatorOpacity: 1);
   }
 
   @override
@@ -620,9 +618,9 @@ class _RenderSegmentedControl<T> extends RenderBox
   // when true some animation tweens will be updated in paint phase.
   bool _needsThumbAnimationUpdate = false;
 
-  int get highlightedIndex => _highlightedIndex;
-  int _highlightedIndex;
-  set highlightedIndex(int value) {
+  int? get highlightedIndex => _highlightedIndex;
+  int? _highlightedIndex;
+  set highlightedIndex(int? value) {
     if (_highlightedIndex == value) {
       return;
     }
@@ -644,16 +642,16 @@ class _RenderSegmentedControl<T> extends RenderBox
     markNeedsSemanticsUpdate();
   }
 
-  void guardedSetHighlightedIndex(int value) {
+  void guardedSetHighlightedIndex(int? value) {
     // Ignore set highlightedIndex when the user is dragging the thumb around.
     if (_startedOnSelectedSegment == true)
       return;
     highlightedIndex = value;
   }
 
-  int get pressedIndex => _pressedIndex;
-  int _pressedIndex;
-  set pressedIndex(int value) {
+  int? get pressedIndex => _pressedIndex;
+  int? _pressedIndex;
+  set pressedIndex(int? value) {
     if (_pressedIndex == value) {
       return;
     }
@@ -664,9 +662,9 @@ class _RenderSegmentedControl<T> extends RenderBox
     state.pressed = state.indexToKey(value);
   }
 
-  Color get thumbColor => _thumbColor;
-  Color _thumbColor;
-  set thumbColor(Color value) {
+  Color? get thumbColor => _thumbColor;
+  Color? _thumbColor;
+  set thumbColor(Color? value) {
     if (_thumbColor == value) {
       return;
     }
@@ -686,13 +684,13 @@ class _RenderSegmentedControl<T> extends RenderBox
     }
   }
 
-  int indexFromLocation(Offset location) {
+  int? indexFromLocation(Offset location) {
     return childCount == 0
       ? null
       // This assumes all children have the same width.
-      : ((location.dx / (size.width / childCount))
+      : (location.dx / (size.width / childCount))
           .floor()
-        .clamp(0, childCount - 1) as int);
+          .clamp(0, childCount - 1);
   }
 
   void _onTapUp(TapUpDetails details) {
@@ -703,20 +701,20 @@ class _RenderSegmentedControl<T> extends RenderBox
   void _onDown(DragDownDetails details) {
     assert(size.contains(details.localPosition));
     _localDragOffset = details.localPosition;
-    final int index = indexFromLocation(_localDragOffset);
+    final int? index = indexFromLocation(_localDragOffset!);
     _startedOnSelectedSegment = index == highlightedIndex;
     pressedIndex = index;
 
-    if (_startedOnSelectedSegment) {
+    if (_startedOnSelectedSegment!) {
       _playThumbScaleAnimation(isExpanding: false);
     }
   }
 
   void _onUpdate(DragUpdateDetails details) {
     _localDragOffset = details.localPosition;
-    final int newIndex = indexFromLocation(_localDragOffset);
+    final int? newIndex = indexFromLocation(_localDragOffset!);
 
-    if (_startedOnSelectedSegment) {
+    if (_startedOnSelectedSegment!) {
       highlightedIndex = newIndex;
       pressedIndex = newIndex;
     } else {
@@ -725,7 +723,7 @@ class _RenderSegmentedControl<T> extends RenderBox
   }
 
   void _onEnd(DragEndDetails details) {
-    if (_startedOnSelectedSegment) {
+    if (_startedOnSelectedSegment!) {
       _playThumbScaleAnimation(isExpanding: true);
       state.didChangeSelectedViaGesture();
     }
@@ -740,7 +738,7 @@ class _RenderSegmentedControl<T> extends RenderBox
   }
 
   void _onCancel() {
-    if (_startedOnSelectedSegment) {
+    if (_startedOnSelectedSegment!) {
       _playThumbScaleAnimation(isExpanding: true);
     }
 
@@ -749,7 +747,7 @@ class _RenderSegmentedControl<T> extends RenderBox
     _startedOnSelectedSegment = null;
   }
 
-  void _playThumbScaleAnimation({ @required bool isExpanding }) {
+  void _playThumbScaleAnimation({ required bool isExpanding }) {
     assert(isExpanding != null);
     _thumbScaleTween = Tween<double>(begin: currentThumbScale, end: isExpanding ? 1 : _kMinThumbScale);
     state.thumbScaleController.animateWith(_kThumbSpringAnimationSimulation);
@@ -762,7 +760,7 @@ class _RenderSegmentedControl<T> extends RenderBox
 
   @override
   double computeMinIntrinsicWidth(double height) {
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     double maxMinChildWidth = 0;
     while (child != null) {
       final _SegmentedControlContainerBoxParentData childParentData =
@@ -776,7 +774,7 @@ class _RenderSegmentedControl<T> extends RenderBox
 
   @override
   double computeMaxIntrinsicWidth(double height) {
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     double maxMaxChildWidth = 0;
     while (child != null) {
       final _SegmentedControlContainerBoxParentData childParentData =
@@ -790,7 +788,7 @@ class _RenderSegmentedControl<T> extends RenderBox
 
   @override
   double computeMinIntrinsicHeight(double width) {
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     double maxMinChildHeight = 0;
     while (child != null) {
       final _SegmentedControlContainerBoxParentData childParentData =
@@ -804,7 +802,7 @@ class _RenderSegmentedControl<T> extends RenderBox
 
   @override
   double computeMaxIntrinsicHeight(double width) {
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     double maxMaxChildHeight = 0;
     while (child != null) {
       final _SegmentedControlContainerBoxParentData childParentData =
@@ -817,7 +815,7 @@ class _RenderSegmentedControl<T> extends RenderBox
   }
 
   @override
-  double computeDistanceToActualBaseline(TextBaseline baseline) {
+  double? computeDistanceToActualBaseline(TextBaseline baseline) {
     return defaultComputeDistanceToHighestActualBaseline(baseline);
   }
 
@@ -843,7 +841,7 @@ class _RenderSegmentedControl<T> extends RenderBox
       (constraints.maxWidth - totalSeparatorWidth) / childCount,
     );
 
-    RenderBox child = firstChild;
+    RenderBox? child = firstChild;
     while (child != null) {
       final double boxHeight = child.getMaxIntrinsicHeight(childWidth);
       maxHeight = math.max(maxHeight, boxHeight);
@@ -890,13 +888,13 @@ class _RenderSegmentedControl<T> extends RenderBox
         for (int i = 0; i < childCount - 1; i += 1) {
           // The separator associated with the last child will not be painted (unless
           // a new trailing segment is added), and its opacity will always be 1.
-          final bool shouldFadeOut = i == highlightedIndex || i == highlightedIndex - 1;
+          final bool shouldFadeOut = i == highlightedIndex || i == highlightedIndex! - 1;
           final RenderBox child = children[i];
-          _childAnimations[child] = _ChildAnimationManifest(separatorOpacity: shouldFadeOut ? 0 : 1);
+          _childAnimations![child] = _ChildAnimationManifest(separatorOpacity: shouldFadeOut ? 0 : 1);
         }
       }
 
-      final RenderBox selectedChild = children[highlightedIndex];
+      final RenderBox selectedChild = children[highlightedIndex!];
 
       final _SegmentedControlContainerBoxParentData childParentData =
         selectedChild.parentData as _SegmentedControlContainerBoxParentData;
@@ -910,9 +908,9 @@ class _RenderSegmentedControl<T> extends RenderBox
         for (int i = 0; i < childCount - 1; i += 1) {
           // The separator associated with the last child will not be painted (unless
           // a new segment is appended to the child list), and its opacity will always be 1.
-          final bool shouldFadeOut = i == highlightedIndex || i == highlightedIndex - 1;
+          final bool shouldFadeOut = i == highlightedIndex || i == highlightedIndex! - 1;
           final RenderBox child = children[i];
-          final _ChildAnimationManifest manifest = _childAnimations[child];
+          final _ChildAnimationManifest manifest = _childAnimations![child]!;
           assert(manifest != null);
           manifest.separatorTween = Tween<double>(
             begin: manifest.separatorOpacity,
@@ -921,8 +919,8 @@ class _RenderSegmentedControl<T> extends RenderBox
         }
 
         _needsThumbAnimationUpdate = false;
-      } else if (_currentThumbTween != null && unscaledThumbTargetRect != _currentThumbTween.begin) {
-        _currentThumbTween = RectTween(begin: _currentThumbTween.begin, end: unscaledThumbTargetRect);
+      } else if (_currentThumbTween != null && unscaledThumbTargetRect != _currentThumbTween!.begin) {
+        _currentThumbTween = RectTween(begin: _currentThumbTween!.begin, end: unscaledThumbTargetRect);
       }
 
       for (int index = 0; index < childCount - 1; index += 1) {
@@ -935,9 +933,9 @@ class _RenderSegmentedControl<T> extends RenderBox
       currentThumbScale = _thumbScaleTween.evaluate(state.thumbScaleController);
 
       final Rect thumbRect = Rect.fromCenter(
-        center: currentThumbRect.center,
-        width: currentThumbRect.width * currentThumbScale,
-        height: currentThumbRect.height * currentThumbScale,
+        center: currentThumbRect!.center,
+        width: currentThumbRect!.width * currentThumbScale,
+        height: currentThumbRect!.height * currentThumbScale,
       );
 
       _paintThumb(context, offset, thumbRect);
@@ -964,8 +962,8 @@ class _RenderSegmentedControl<T> extends RenderBox
 
     final Paint paint = Paint();
 
-    final _ChildAnimationManifest manifest = _childAnimations == null ? null : _childAnimations[child];
-    final double opacity = manifest?.separatorTween?.evaluate(state.separatorOpacityController) ?? 1;
+    final _ChildAnimationManifest? manifest = _childAnimations == null ? null : _childAnimations![child];
+    final double opacity = manifest?.separatorTween.evaluate(state.separatorOpacityController) ?? 1;
     manifest?.separatorOpacity = opacity;
     paint.color = _kSeparatorColor.withOpacity(_kSeparatorColor.opacity * opacity);
 
@@ -1015,14 +1013,14 @@ class _RenderSegmentedControl<T> extends RenderBox
 
     context.canvas.drawRRect(
       thumbRRect,
-      Paint()..color = thumbColor,
+      Paint()..color = thumbColor!,
     );
   }
 
   @override
-  bool hitTestChildren(BoxHitTestResult result, { @required Offset position }) {
+  bool hitTestChildren(BoxHitTestResult result, { required Offset position }) {
     assert(position != null);
-    RenderBox child = lastChild;
+    RenderBox? child = lastChild;
     while (child != null) {
       final _SegmentedControlContainerBoxParentData childParentData =
         child.parentData as _SegmentedControlContainerBoxParentData;
@@ -1032,7 +1030,7 @@ class _RenderSegmentedControl<T> extends RenderBox
           position: position,
           hitTest: (BoxHitTestResult result, Offset localOffset) {
             assert(localOffset == position - childParentData.offset);
-            return child.hitTest(result, position: localOffset);
+            return child!.hitTest(result, position: localOffset);
           },
         );
       }

packages/flutter/lib/src/cupertino/tab_scaffold.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';
 import 'bottom_tab_bar.dart';
@@ -207,9 +205,9 @@ class CupertinoTabScaffold extends StatefulWidget {
   ///
   /// The [tabBar] and [tabBuilder] arguments must not be null.
   CupertinoTabScaffold({
-    Key key,
-    @required this.tabBar,
-    @required this.tabBuilder,
+    Key? key,
+    required this.tabBar,
+    required this.tabBuilder,
     this.controller,
     this.backgroundColor,
     this.resizeToAvoidBottomInset = true,
@@ -258,7 +256,7 @@ class CupertinoTabScaffold extends StatefulWidget {
   /// index value.
   ///
   /// Defaults to null.
-  final CupertinoTabController controller;
+  final CupertinoTabController? controller;
 
   /// An [IndexedWidgetBuilder] that's called when tabs become active.
   ///
@@ -280,7 +278,7 @@ class CupertinoTabScaffold extends StatefulWidget {
   /// The color of the widget that underlies the entire scaffold.
   ///
   /// By default uses [CupertinoTheme]'s `scaffoldBackgroundColor` when null.
-  final Color backgroundColor;
+  final Color? backgroundColor;
 
   /// Whether the body should size itself to avoid the window's bottom inset.
   ///
@@ -296,7 +294,7 @@ class CupertinoTabScaffold extends StatefulWidget {
 }
 
 class _CupertinoTabScaffoldState extends State<CupertinoTabScaffold> {
-  CupertinoTabController _controller;
+  CupertinoTabController? _controller;
 
   @override
   void initState() {
@@ -317,7 +315,7 @@ class _CupertinoTabScaffoldState extends State<CupertinoTabScaffold> {
     if (shouldDisposeOldController) {
       _controller?.dispose();
     } else if (_controller?._isDisposed == false) {
-      _controller.removeListener(_onCurrentIndexChange);
+      _controller!.removeListener(_onCurrentIndexChange);
     }
 
     newController.addListener(_onCurrentIndexChange);
@@ -326,8 +324,8 @@ class _CupertinoTabScaffoldState extends State<CupertinoTabScaffold> {
 
   void _onCurrentIndexChange() {
     assert(
-      _controller.index >= 0 && _controller.index < widget.tabBar.items.length,
-      "The $runtimeType's current index ${_controller.index} is "
+      _controller!.index >= 0 && _controller!.index < widget.tabBar.items.length,
+      "The $runtimeType's current index ${_controller!.index} is "
       'out of bounds for the tab bar with ${widget.tabBar.items.length} tabs'
     );
 
@@ -341,20 +339,20 @@ class _CupertinoTabScaffoldState extends State<CupertinoTabScaffold> {
     super.didUpdateWidget(oldWidget);
     if (widget.controller != oldWidget.controller) {
       _updateTabController(shouldDisposeOldController: oldWidget.controller == null);
-    } else if (_controller.index >= widget.tabBar.items.length) {
+    } else if (_controller!.index >= widget.tabBar.items.length) {
       // If a new [tabBar] with less than (_controller.index + 1) items is provided,
       // clamp the current index.
-      _controller.index = widget.tabBar.items.length - 1;
+      _controller!.index = widget.tabBar.items.length - 1;
     }
   }
 
   @override
   Widget build(BuildContext context) {
-    final MediaQueryData existingMediaQuery = MediaQuery.of(context);
-    MediaQueryData newMediaQuery = MediaQuery.of(context);
+    final MediaQueryData existingMediaQuery = MediaQuery.of(context)!;
+    MediaQueryData newMediaQuery = MediaQuery.of(context)!;
 
     Widget content = _TabSwitchingView(
-      currentTabIndex: _controller.index,
+      currentTabIndex: _controller!.index,
       tabCount: widget.tabBar.items.length,
       tabBuilder: widget.tabBuilder,
     );
@@ -417,12 +415,11 @@ class _CupertinoTabScaffoldState extends State<CupertinoTabScaffold> {
               // our own listener to update the [_controller.currentIndex] on top of a possibly user
               // provided callback.
               child: widget.tabBar.copyWith(
-                currentIndex: _controller.index,
+                currentIndex: _controller!.index,
                 onTap: (int newIndex) {
-                  _controller.index = newIndex;
+                  _controller!.index = newIndex;
                   // Chain the user's original callback.
-                  if (widget.tabBar.onTap != null)
-                    widget.tabBar.onTap(newIndex);
+                  widget.tabBar.onTap?.call(newIndex);
                 },
               ),
             ),
@@ -438,7 +435,7 @@ class _CupertinoTabScaffoldState extends State<CupertinoTabScaffold> {
     if (widget.controller == null) {
       _controller?.dispose();
     } else if (_controller?._isDisposed == false) {
-      _controller.removeListener(_onCurrentIndexChange);
+      _controller!.removeListener(_onCurrentIndexChange);
     }
 
     super.dispose();
@@ -449,9 +446,9 @@ class _CupertinoTabScaffoldState extends State<CupertinoTabScaffold> {
 /// at a time and on stage. Off stage tabs' animations are stopped.
 class _TabSwitchingView extends StatefulWidget {
   const _TabSwitchingView({
-    @required this.currentTabIndex,
-    @required this.tabCount,
-    @required this.tabBuilder,
+    required this.currentTabIndex,
+    required this.tabCount,
+    required this.tabBuilder,
   }) : assert(currentTabIndex != null),
        assert(tabCount != null && tabCount > 0),
        assert(tabBuilder != null);

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

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'package:flutter/widgets.dart';
 
 import 'app.dart' show CupertinoApp;
@@ -43,7 +41,7 @@ import 'route.dart';
 class CupertinoTabView extends StatefulWidget {
   /// Creates the content area for a tab in a [CupertinoTabScaffold].
   const CupertinoTabView({
-    Key key,
+    Key? key,
     this.builder,
     this.navigatorKey,
     this.defaultTitle,
@@ -68,7 +66,7 @@ class CupertinoTabView extends StatefulWidget {
   ///  * Calling [State.setState] on a descendant [StatefulWidget]'s [State]
   ///  * Modifying an [InheritedWidget] that a descendant registered itself
   ///    as a dependent to.
-  final WidgetBuilder builder;
+  final WidgetBuilder? builder;
 
   /// A key to use when building this widget's [Navigator].
   ///
@@ -81,10 +79,10 @@ class CupertinoTabView extends StatefulWidget {
   /// tab's state in the process; in that case, the [navigatorObservers]
   /// must also be changed, since the previous observers will be attached to the
   /// previous navigator.
-  final GlobalKey<NavigatorState> navigatorKey;
+  final GlobalKey<NavigatorState>? navigatorKey;
 
   /// The title of the default route.
-  final String defaultTitle;
+  final String? defaultTitle;
 
   /// This tab view's routing table.
   ///
@@ -105,12 +103,12 @@ class CupertinoTabView extends StatefulWidget {
   ///
   /// This routing table is not shared with any routing tables of ancestor or
   /// descendant [Navigator]s.
-  final Map<String, WidgetBuilder> routes;
+  final Map<String, WidgetBuilder>? routes;
 
   /// The route generator callback used when the tab view is navigated to a named route.
   ///
   /// This is used if [routes] does not contain the requested route.
-  final RouteFactory onGenerateRoute;
+  final RouteFactory? onGenerateRoute;
 
   /// Called when [onGenerateRoute] also fails to generate a route.
   ///
@@ -120,7 +118,7 @@ class CupertinoTabView extends StatefulWidget {
   ///
   /// The default implementation pushes a route that displays an ugly error
   /// message.
-  final RouteFactory onUnknownRoute;
+  final RouteFactory? onUnknownRoute;
 
   /// The list of observers for the [Navigator] created in this tab view.
   ///
@@ -134,8 +132,8 @@ class CupertinoTabView extends StatefulWidget {
 }
 
 class _CupertinoTabViewState extends State<CupertinoTabView> {
-  HeroController _heroController;
-  List<NavigatorObserver> _navigatorObservers;
+  late HeroController _heroController;
+  late List<NavigatorObserver> _navigatorObservers;
 
   @override
   void initState() {
@@ -169,15 +167,15 @@ class _CupertinoTabViewState extends State<CupertinoTabView> {
     );
   }
 
-  Route<dynamic> _onGenerateRoute(RouteSettings settings) {
-    final String name = settings.name;
-    WidgetBuilder routeBuilder;
-    String title;
+  Route<dynamic>? _onGenerateRoute(RouteSettings settings) {
+    final String? name = settings.name;
+    WidgetBuilder? routeBuilder;
+    String? title;
     if (name == Navigator.defaultRouteName && widget.builder != null) {
-      routeBuilder = widget.builder;
+      routeBuilder = widget.builder!;
       title = widget.defaultTitle;
     } else if (widget.routes != null) {
-      routeBuilder = widget.routes[name];
+      routeBuilder = widget.routes![name];
     }
     if (routeBuilder != null) {
       return CupertinoPageRoute<dynamic>(
@@ -187,11 +185,11 @@ class _CupertinoTabViewState extends State<CupertinoTabView> {
       );
     }
     if (widget.onGenerateRoute != null)
-      return widget.onGenerateRoute(settings);
+      return widget.onGenerateRoute!(settings);
     return null;
   }
 
-  Route<dynamic> _onUnknownRoute(RouteSettings settings) {
+  Route<dynamic>? _onUnknownRoute(RouteSettings settings) {
     assert(() {
       if (widget.onUnknownRoute == null) {
         throw FlutterError(
@@ -208,7 +206,7 @@ class _CupertinoTabViewState extends State<CupertinoTabView> {
       }
       return true;
     }());
-    final Route<dynamic> result = widget.onUnknownRoute(settings);
+    final Route<dynamic>? result = widget.onUnknownRoute!(settings);
     assert(() {
       if (result == null) {
         throw FlutterError(

packages/flutter/lib/src/cupertino/text_field.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' as ui show BoxHeightStyle, BoxWidthStyle;
 
 import 'package:flutter/gestures.dart';
@@ -89,7 +87,7 @@ enum OverlayVisibilityMode {
 
 class _CupertinoTextFieldSelectionGestureDetectorBuilder extends TextSelectionGestureDetectorBuilder {
   _CupertinoTextFieldSelectionGestureDetectorBuilder({
-    @required _CupertinoTextFieldState state,
+    required _CupertinoTextFieldState state,
   }) : _state = state,
        super(delegate: state);
 
@@ -102,7 +100,7 @@ class _CupertinoTextFieldSelectionGestureDetectorBuilder extends TextSelectionGe
     // this handler. If the clear button widget recognizes the up event,
     // then do not handle it.
     if (_state._clearGlobalKey.currentContext != null) {
-      final RenderBox renderBox = _state._clearGlobalKey.currentContext.findRenderObject() as RenderBox;
+      final RenderBox renderBox = _state._clearGlobalKey.currentContext!.findRenderObject() as RenderBox;
       final Offset localOffset = renderBox.globalToLocal(details.globalPosition);
       if (renderBox.hitTest(BoxHitTestResult(), position: localOffset)) {
         return;
@@ -111,7 +109,7 @@ class _CupertinoTextFieldSelectionGestureDetectorBuilder extends TextSelectionGe
     super.onSingleTapUp(details);
     _state._requestKeyboard();
     if (_state.widget.onTap != null)
-      _state.widget.onTap();
+      _state.widget.onTap!();
   }
 
   @override
@@ -218,7 +216,7 @@ class CupertinoTextField extends StatefulWidget {
   ///  * [maxLength], which discusses the precise meaning of "number of
   ///    characters" and how it may differ from the intuitive meaning.
   const CupertinoTextField({
-    Key key,
+    Key? key,
     this.controller,
     this.focusNode,
     this.decoration = _kDefaultRoundedBorderDecoration,
@@ -233,7 +231,7 @@ class CupertinoTextField extends StatefulWidget {
     this.suffix,
     this.suffixMode = OverlayVisibilityMode.always,
     this.clearButtonMode = OverlayVisibilityMode.never,
-    TextInputType keyboardType,
+    TextInputType? keyboardType,
     this.textInputAction,
     this.textCapitalization = TextCapitalization.none,
     this.style,
@@ -241,14 +239,14 @@ class CupertinoTextField extends StatefulWidget {
     this.textAlign = TextAlign.start,
     this.textAlignVertical,
     this.readOnly = false,
-    ToolbarOptions toolbarOptions,
+    ToolbarOptions? toolbarOptions,
     this.showCursor,
     this.autofocus = false,
     this.obscuringCharacter = '•',
     this.obscureText = false,
     this.autocorrect = true,
-    SmartDashesType smartDashesType,
-    SmartQuotesType smartQuotesType,
+    SmartDashesType? smartDashesType,
+    SmartQuotesType? smartQuotesType,
     this.enableSuggestions = true,
     this.maxLines = 1,
     this.minLines,
@@ -278,7 +276,8 @@ class CupertinoTextField extends StatefulWidget {
   }) : assert(textAlign != null),
        assert(readOnly != null),
        assert(autofocus != null),
-       assert(obscuringCharacter != null && obscuringCharacter.length == 1),
+       // TODO(a14n): uncomment when issue is fixed, https://github.com/dart-lang/sdk/issues/43407
+       assert(obscuringCharacter != null/* && obscuringCharacter.length == 1*/),
        assert(obscureText != null),
        assert(autocorrect != null),
        smartDashesType = smartDashesType ?? (obscureText ? SmartDashesType.disabled : SmartDashesType.enabled),
@@ -327,16 +326,16 @@ class CupertinoTextField extends StatefulWidget {
   /// Controls the text being edited.
   ///
   /// If null, this widget will create its own [TextEditingController].
-  final TextEditingController controller;
+  final TextEditingController? controller;
 
   /// {@macro flutter.widgets.Focus.focusNode}
-  final FocusNode focusNode;
+  final FocusNode? focusNode;
 
   /// Controls the [BoxDecoration] of the box behind the text input.
   ///
   /// Defaults to having a rounded rectangle grey border and can be null to have
   /// no box decoration.
-  final BoxDecoration decoration;
+  final BoxDecoration? decoration;
 
   /// Padding around the text entry area between the [prefix] and [suffix]
   /// or the clear button when [clearButtonMode] is not never.
@@ -351,7 +350,7 @@ class CupertinoTextField extends StatefulWidget {
   ///
   /// The text style of the placeholder text matches that of the text field's
   /// main text entry except a lighter font weight and a grey font color.
-  final String placeholder;
+  final String? placeholder;
 
   /// The style to use for the placeholder text.
   ///
@@ -362,10 +361,10 @@ class CupertinoTextField extends StatefulWidget {
   /// Defaults to the [style] property with w300 font weight and grey color.
   ///
   /// If specifically set to null, placeholder's style will be the same as [style].
-  final TextStyle placeholderStyle;
+  final TextStyle? placeholderStyle;
 
   /// An optional [Widget] to display before the text.
-  final Widget prefix;
+  final Widget? prefix;
 
   /// Controls the visibility of the [prefix] widget based on the state of
   /// text entry when the [prefix] argument is not null.
@@ -376,7 +375,7 @@ class CupertinoTextField extends StatefulWidget {
   final OverlayVisibilityMode prefixMode;
 
   /// An optional [Widget] to display after the text.
-  final Widget suffix;
+  final Widget? suffix;
 
   /// Controls the visibility of the [suffix] widget based on the state of
   /// text entry when the [suffix] argument is not null.
@@ -403,7 +402,7 @@ class CupertinoTextField extends StatefulWidget {
   ///
   /// Defaults to [TextInputAction.newline] if [keyboardType] is
   /// [TextInputType.multiline] and [TextInputAction.done] otherwise.
-  final TextInputAction textInputAction;
+  final TextInputAction? textInputAction;
 
   /// {@macro flutter.widgets.editableText.textCapitalization}
   final TextCapitalization textCapitalization;
@@ -413,10 +412,10 @@ class CupertinoTextField extends StatefulWidget {
   /// Also serves as a base for the [placeholder] text's style.
   ///
   /// Defaults to the standard iOS font style from [CupertinoTheme] if null.
-  final TextStyle style;
+  final TextStyle? style;
 
   /// {@macro flutter.widgets.editableText.strutStyle}
-  final StrutStyle strutStyle;
+  final StrutStyle? strutStyle;
 
   /// {@macro flutter.widgets.editableText.textAlign}
   final TextAlign textAlign;
@@ -429,13 +428,13 @@ class CupertinoTextField extends StatefulWidget {
   final ToolbarOptions toolbarOptions;
 
   /// {@macro flutter.widgets.inputDecorator.textAlignVertical}
-  final TextAlignVertical textAlignVertical;
+  final TextAlignVertical? textAlignVertical;
 
   /// {@macro flutter.widgets.editableText.readOnly}
   final bool readOnly;
 
   /// {@macro flutter.widgets.editableText.showCursor}
-  final bool showCursor;
+  final bool? showCursor;
 
   /// {@macro flutter.widgets.editableText.autofocus}
   final bool autofocus;
@@ -459,10 +458,10 @@ class CupertinoTextField extends StatefulWidget {
   final bool enableSuggestions;
 
   /// {@macro flutter.widgets.editableText.maxLines}
-  final int maxLines;
+  final int? maxLines;
 
   /// {@macro flutter.widgets.editableText.minLines}
-  final int minLines;
+  final int? minLines;
 
   /// {@macro flutter.widgets.editableText.expands}
   final bool expands;
@@ -509,7 +508,7 @@ class CupertinoTextField extends StatefulWidget {
   ///
   ///  * [LengthLimitingTextInputFormatter] for more information on how it
   ///    counts characters, and how it may differ from the intuitive meaning.
-  final int maxLength;
+  final int? maxLength;
 
   /// If true, prevents the field from allowing more than [maxLength]
   /// characters.
@@ -520,10 +519,10 @@ class CupertinoTextField extends StatefulWidget {
   final bool maxLengthEnforced;
 
   /// {@macro flutter.widgets.editableText.onChanged}
-  final ValueChanged<String> onChanged;
+  final ValueChanged<String>? onChanged;
 
   /// {@macro flutter.widgets.editableText.onEditingComplete}
-  final VoidCallback onEditingComplete;
+  final VoidCallback? onEditingComplete;
 
   /// {@macro flutter.widgets.editableText.onSubmitted}
   ///
@@ -532,23 +531,23 @@ class CupertinoTextField extends StatefulWidget {
   ///  * [EditableText.onSubmitted] for an example of how to handle moving to
   ///    the next/previous field when using [TextInputAction.next] and
   ///    [TextInputAction.previous] for [textInputAction].
-  final ValueChanged<String> onSubmitted;
+  final ValueChanged<String>? onSubmitted;
 
   /// {@macro flutter.widgets.editableText.inputFormatters}
-  final List<TextInputFormatter> inputFormatters;
+  final List<TextInputFormatter>? inputFormatters;
 
   /// Disables the text field when false.
   ///
   /// Text fields in disabled states have a light grey background and don't
   /// respond to touch events including the [prefix], [suffix] and the clear
   /// button.
-  final bool enabled;
+  final bool? enabled;
 
   /// {@macro flutter.widgets.editableText.cursorWidth}
   final double cursorWidth;
 
   /// {@macro flutter.widgets.editableText.cursorHeight}
-  final double cursorHeight;
+  final double? cursorHeight;
 
   /// {@macro flutter.widgets.editableText.cursorRadius}
   final Radius cursorRadius;
@@ -558,7 +557,7 @@ class CupertinoTextField extends StatefulWidget {
   /// Defaults to the [CupertinoThemeData.primaryColor] of the ambient theme,
   /// which itself defaults to [CupertinoColors.activeBlue] in the light theme
   /// and [CupertinoColors.activeOrange] in the dark theme.
-  final Color cursorColor;
+  final Color? cursorColor;
 
   /// Controls how tall the selection highlight boxes are computed to be.
   ///
@@ -575,7 +574,7 @@ class CupertinoTextField extends StatefulWidget {
   /// This setting is only honored on iOS devices.
   ///
   /// If null, defaults to [Brightness.light].
-  final Brightness keyboardAppearance;
+  final Brightness? keyboardAppearance;
 
   /// {@macro flutter.widgets.editableText.scrollPadding}
   final EdgeInsets scrollPadding;
@@ -587,23 +586,23 @@ class CupertinoTextField extends StatefulWidget {
   final DragStartBehavior dragStartBehavior;
 
   /// {@macro flutter.widgets.editableText.scrollController}
-  final ScrollController scrollController;
+  final ScrollController? scrollController;
 
   /// {@macro flutter.widgets.editableText.scrollPhysics}
-  final ScrollPhysics scrollPhysics;
+  final ScrollPhysics? scrollPhysics;
 
   /// {@macro flutter.widgets.editableText.selectionEnabled}
   bool get selectionEnabled => enableInteractiveSelection;
 
   /// {@macro flutter.material.textfield.onTap}
-  final GestureTapCallback onTap;
+  final GestureTapCallback? onTap;
 
   /// {@macro flutter.widgets.editableText.autofillHints}
   /// {@macro flutter.services.autofill.autofillHints}
-  final Iterable<String> autofillHints;
+  final Iterable<String>? autofillHints;
 
   /// {@macro flutter.material.textfield.restorationId}
-  final String restorationId;
+  final String? restorationId;
 
   @override
   _CupertinoTextFieldState createState() => _CupertinoTextFieldState();
@@ -649,15 +648,15 @@ class CupertinoTextField extends StatefulWidget {
 class _CupertinoTextFieldState extends State<CupertinoTextField> with RestorationMixin, AutomaticKeepAliveClientMixin<CupertinoTextField> implements TextSelectionGestureDetectorBuilderDelegate {
   final GlobalKey _clearGlobalKey = GlobalKey();
 
-  RestorableTextEditingController _controller;
-  TextEditingController get _effectiveController => widget.controller ?? _controller.value;
+  RestorableTextEditingController? _controller;
+  TextEditingController get _effectiveController => widget.controller ?? _controller!.value;
 
-  FocusNode _focusNode;
+  FocusNode? _focusNode;
   FocusNode get _effectiveFocusNode => widget.focusNode ?? (_focusNode ??= FocusNode());
 
   bool _showSelectionHandles = false;
 
-  _CupertinoTextFieldSelectionGestureDetectorBuilder _selectionGestureDetectorBuilder;
+  late _CupertinoTextFieldSelectionGestureDetectorBuilder _selectionGestureDetectorBuilder;
 
   // API for TextSelectionGestureDetectorBuilderDelegate.
   @override
@@ -683,10 +682,10 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
   void didUpdateWidget(CupertinoTextField oldWidget) {
     super.didUpdateWidget(oldWidget);
     if (widget.controller == null && oldWidget.controller != null) {
-      _createLocalController(oldWidget.controller.value);
+      _createLocalController(oldWidget.controller!.value);
     } else if (widget.controller != null && oldWidget.controller == null) {
-      unregisterFromRestoration(_controller);
-      _controller.dispose();
+      unregisterFromRestoration(_controller!);
+      _controller!.dispose();
       _controller = null;
     }
     final bool isEnabled = widget.enabled ?? true;
@@ -697,7 +696,7 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
   }
 
   @override
-  void restoreState(RestorationBucket oldBucket, bool initialRestore) {
+  void restoreState(RestorationBucket? oldBucket, bool initialRestore) {
     if (_controller != null) {
       _registerController();
     }
@@ -705,11 +704,11 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
 
   void _registerController() {
     assert(_controller != null);
-    registerForRestoration(_controller, 'controller');
-    _controller.value.addListener(updateKeepAlive);
+    registerForRestoration(_controller!, 'controller');
+    _controller!.value.addListener(updateKeepAlive);
   }
 
-  void _createLocalController([TextEditingValue value]) {
+  void _createLocalController([TextEditingValue? value]) {
     assert(_controller == null);
     _controller = value == null
         ? RestorableTextEditingController()
@@ -720,7 +719,7 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
   }
 
   @override
-  String get restorationId => widget.restorationId;
+  String? get restorationId => widget.restorationId;
 
   @override
   void dispose() {
@@ -729,13 +728,13 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
     super.dispose();
   }
 
-  EditableTextState get _editableText => editableTextKey.currentState;
+  EditableTextState? get _editableText => editableTextKey.currentState;
 
   void _requestKeyboard() {
     _editableText?.requestKeyboard();
   }
 
-  bool _shouldShowSelectionHandles(SelectionChangedCause cause) {
+  bool _shouldShowSelectionHandles(SelectionChangedCause? cause) {
     // When the text field is activated by something that doesn't trigger the
     // selection overlay, we shouldn't show the handles either.
     if (!_selectionGestureDetectorBuilder.shouldShowSelectionToolbar)
@@ -754,7 +753,7 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
     return false;
   }
 
-  void _handleSelectionChanged(TextSelection selection, SelectionChangedCause cause) {
+  void _handleSelectionChanged(TextSelection selection, SelectionChangedCause? cause) {
     if (cause == SelectionChangedCause.longPress) {
       _editableText?.bringIntoView(selection.base);
     }
@@ -767,11 +766,11 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
   }
 
   @override
-  bool get wantKeepAlive => _controller?.value?.text?.isNotEmpty == true;
+  bool get wantKeepAlive => _controller?.value.text.isNotEmpty == true;
 
   bool _shouldShowAttachment({
-    OverlayVisibilityMode attachment,
-    bool hasText,
+    required OverlayVisibilityMode attachment,
+    required bool hasText,
   }) {
     switch (attachment) {
       case OverlayVisibilityMode.never:
@@ -783,8 +782,6 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
       case OverlayVisibilityMode.notEditing:
         return !hasText;
     }
-    assert(false);
-    return null;
   }
 
   bool _showPrefixWidget(TextEditingValue text) {
@@ -821,7 +818,7 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
   // like a prefix or suffix, in which case it's aligned to the center.
   TextAlignVertical get _textAlignVertical {
     if (widget.textAlignVertical != null) {
-      return widget.textAlignVertical;
+      return widget.textAlignVertical!;
     }
     return _hasDecoration ? TextAlignVertical.center : TextAlignVertical.top;
   }
@@ -840,11 +837,11 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
     return ValueListenableBuilder<TextEditingValue>(
       valueListenable: _effectiveController,
       child: editableText,
-      builder: (BuildContext context, TextEditingValue text, Widget child) {
+      builder: (BuildContext context, TextEditingValue? text, Widget? child) {
         return Row(children: <Widget>[
           // Insert a prefix at the front if the prefix visibility mode matches
           // the current text state.
-          if (_showPrefixWidget(text)) widget.prefix,
+          if (_showPrefixWidget(text!)) widget.prefix!,
           // In the middle part, stack the placeholder on top of the main EditableText
           // if needed.
           Expanded(
@@ -856,7 +853,7 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
                     child: Padding(
                       padding: widget.padding,
                       child: Text(
-                        widget.placeholder,
+                        widget.placeholder!,
                         maxLines: widget.maxLines,
                         overflow: TextOverflow.ellipsis,
                         style: placeholderStyle,
@@ -864,13 +861,13 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
                       ),
                     ),
                   ),
-                child,
+                child!,
               ],
             ),
           ),
           // First add the explicit suffix if the suffix visibility mode matches.
           if (_showSuffixWidget(text))
-            widget.suffix
+            widget.suffix!
           // Otherwise, try to show a clear button if its visibility mode matches.
           else if (_showClearButton(text))
             GestureDetector(
@@ -881,7 +878,7 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
                 final bool textChanged = _effectiveController.text.isNotEmpty;
                 _effectiveController.clear();
                 if (widget.onChanged != null && textChanged)
-                  widget.onChanged(_effectiveController.text);
+                  widget.onChanged!(_effectiveController.text);
               } : null,
               child: Padding(
                 padding: const EdgeInsets.symmetric(horizontal: 6.0),
@@ -904,33 +901,33 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
     final TextEditingController controller = _effectiveController;
     final List<TextInputFormatter> formatters = widget.inputFormatters ?? <TextInputFormatter>[];
     final bool enabled = widget.enabled ?? true;
-    final Offset cursorOffset = Offset(_iOSHorizontalCursorOffsetPixels / MediaQuery.of(context).devicePixelRatio, 0);
+    final Offset cursorOffset = Offset(_iOSHorizontalCursorOffsetPixels / MediaQuery.of(context)!.devicePixelRatio, 0);
     if (widget.maxLength != null && widget.maxLengthEnforced) {
       formatters.add(LengthLimitingTextInputFormatter(widget.maxLength));
     }
     final CupertinoThemeData themeData = CupertinoTheme.of(context);
 
-    final TextStyle resolvedStyle = widget.style?.copyWith(
+    final TextStyle? resolvedStyle = widget.style?.copyWith(
       color: CupertinoDynamicColor.resolve(widget.style?.color, context),
       backgroundColor: CupertinoDynamicColor.resolve(widget.style?.backgroundColor, context),
     );
 
     final TextStyle textStyle = themeData.textTheme.textStyle.merge(resolvedStyle);
 
-    final TextStyle resolvedPlaceholderStyle = widget.placeholderStyle?.copyWith(
+    final TextStyle? resolvedPlaceholderStyle = widget.placeholderStyle?.copyWith(
       color: CupertinoDynamicColor.resolve(widget.placeholderStyle?.color, context),
       backgroundColor: CupertinoDynamicColor.resolve(widget.placeholderStyle?.backgroundColor, context),
     );
 
     final TextStyle placeholderStyle = textStyle.merge(resolvedPlaceholderStyle);
 
-    final Brightness keyboardAppearance = widget.keyboardAppearance ?? CupertinoTheme.brightnessOf(context);
+    final Brightness keyboardAppearance = widget.keyboardAppearance ?? CupertinoTheme.brightnessOf(context)!;
     final Color cursorColor = CupertinoDynamicColor.resolve(widget.cursorColor, context) ?? themeData.primaryColor;
-    final Color disabledColor = CupertinoDynamicColor.resolve(_kDisabledBackground, context);
+    final Color? disabledColor = CupertinoDynamicColor.resolve(_kDisabledBackground, context);
 
-    final Color decorationColor = CupertinoDynamicColor.resolve(widget.decoration?.color, context);
+    final Color? decorationColor = CupertinoDynamicColor.resolve(widget.decoration?.color, context);
 
-    final BoxBorder border = widget.decoration?.border;
+    final BoxBorder? border = widget.decoration?.border;
     Border resolvedBorder = border as Border;
     if (border is Border) {
       BorderSide resolveBorderSide(BorderSide side) {
@@ -948,7 +945,7 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
         );
     }
 
-    final BoxDecoration effectiveDecoration = widget.decoration?.copyWith(
+    final BoxDecoration? effectiveDecoration = widget.decoration?.copyWith(
       border: resolvedBorder,
       color: enabled ? decorationColor : (decorationColor ?? disabledColor),
     );
@@ -1001,7 +998,7 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
             cursorOffset: cursorOffset,
             paintCursorAboveText: true,
             autocorrectionTextRectColor: selectionColor,
-            backgroundCursorColor: CupertinoDynamicColor.resolve(CupertinoColors.inactiveGray, context),
+            backgroundCursorColor: CupertinoDynamicColor.resolve(CupertinoColors.inactiveGray, context)!,
             selectionHeightStyle: widget.selectionHeightStyle,
             selectionWidthStyle: widget.selectionWidthStyle,
             scrollPadding: widget.scrollPadding,

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

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:collection';
 import 'dart:math' as math;
 import 'dart:ui' as ui;
@@ -68,24 +66,24 @@ const EdgeInsets _kToolbarButtonPadding = EdgeInsets.symmetric(vertical: 10.0, h
 // Generates the child that's passed into CupertinoTextSelectionToolbar.
 class _CupertinoTextSelectionToolbarWrapper extends StatefulWidget {
   const _CupertinoTextSelectionToolbarWrapper({
-    Key key,
-    this.arrowTipX,
-    this.barTopY,
+    Key? key,
+    required this.arrowTipX,
+    required this.barTopY,
     this.clipboardStatus,
     this.handleCut,
     this.handleCopy,
     this.handlePaste,
     this.handleSelectAll,
-    this.isArrowPointingDown,
+    required this.isArrowPointingDown,
   }) : super(key: key);
 
   final double arrowTipX;
   final double barTopY;
-  final ClipboardStatusNotifier clipboardStatus;
-  final VoidCallback handleCut;
-  final VoidCallback handleCopy;
-  final VoidCallback handlePaste;
-  final VoidCallback handleSelectAll;
+  final ClipboardStatusNotifier? clipboardStatus;
+  final VoidCallback? handleCut;
+  final VoidCallback? handleCopy;
+  final VoidCallback? handlePaste;
+  final VoidCallback? handleSelectAll;
   final bool isArrowPointingDown;
 
   @override
@@ -93,7 +91,7 @@ class _CupertinoTextSelectionToolbarWrapper extends StatefulWidget {
 }
 
 class _CupertinoTextSelectionToolbarWrapperState extends State<_CupertinoTextSelectionToolbarWrapper> {
-  ClipboardStatusNotifier _clipboardStatus;
+  late ClipboardStatusNotifier _clipboardStatus;
 
   void _onChangedClipboardStatus() {
     setState(() {
@@ -115,16 +113,16 @@ class _CupertinoTextSelectionToolbarWrapperState extends State<_CupertinoTextSel
     if (oldWidget.clipboardStatus == null && widget.clipboardStatus != null) {
       _clipboardStatus.removeListener(_onChangedClipboardStatus);
       _clipboardStatus.dispose();
-      _clipboardStatus = widget.clipboardStatus;
+      _clipboardStatus = widget.clipboardStatus!;
     } else if (oldWidget.clipboardStatus != null) {
       if (widget.clipboardStatus == null) {
         _clipboardStatus = ClipboardStatusNotifier();
         _clipboardStatus.addListener(_onChangedClipboardStatus);
-        oldWidget.clipboardStatus.removeListener(_onChangedClipboardStatus);
+        oldWidget.clipboardStatus!.removeListener(_onChangedClipboardStatus);
       } else if (widget.clipboardStatus != oldWidget.clipboardStatus) {
-        _clipboardStatus = widget.clipboardStatus;
+        _clipboardStatus = widget.clipboardStatus!;
         _clipboardStatus.addListener(_onChangedClipboardStatus);
-        oldWidget.clipboardStatus.removeListener(_onChangedClipboardStatus);
+        oldWidget.clipboardStatus!.removeListener(_onChangedClipboardStatus);
       }
     }
     if (widget.handlePaste != null) {
@@ -154,12 +152,12 @@ class _CupertinoTextSelectionToolbarWrapperState extends State<_CupertinoTextSel
     }
 
     final List<Widget> items = <Widget>[];
-    final CupertinoLocalizations localizations = CupertinoLocalizations.of(context);
+    final CupertinoLocalizations? localizations = CupertinoLocalizations.of(context);
     final EdgeInsets arrowPadding = widget.isArrowPointingDown
       ? EdgeInsets.only(bottom: _kToolbarArrowSize.height)
       : EdgeInsets.only(top: _kToolbarArrowSize.height);
     final Widget onePhysicalPixelVerticalDivider =
-        SizedBox(width: 1.0 / MediaQuery.of(context).devicePixelRatio);
+        SizedBox(width: 1.0 / MediaQuery.of(context)!.devicePixelRatio);
 
     void addToolbarButton(
       String text,
@@ -185,17 +183,17 @@ class _CupertinoTextSelectionToolbarWrapperState extends State<_CupertinoTextSel
     }
 
     if (widget.handleCut != null) {
-      addToolbarButton(localizations.cutButtonLabel, widget.handleCut);
+      addToolbarButton(localizations!.cutButtonLabel, widget.handleCut!);
     }
     if (widget.handleCopy != null) {
-      addToolbarButton(localizations.copyButtonLabel, widget.handleCopy);
+      addToolbarButton(localizations!.copyButtonLabel, widget.handleCopy!);
     }
     if (widget.handlePaste != null
         && _clipboardStatus.value == ClipboardStatus.pasteable) {
-      addToolbarButton(localizations.pasteButtonLabel, widget.handlePaste);
+      addToolbarButton(localizations!.pasteButtonLabel, widget.handlePaste!);
     }
     if (widget.handleSelectAll != null) {
-      addToolbarButton(localizations.selectAllButtonLabel, widget.handleSelectAll);
+      addToolbarButton(localizations!.selectAllButtonLabel, widget.handleSelectAll!);
     }
 
     return CupertinoTextSelectionToolbar._(
@@ -221,11 +219,11 @@ class _CupertinoTextSelectionToolbarWrapperState extends State<_CupertinoTextSel
 @visibleForTesting
 class CupertinoTextSelectionToolbar extends SingleChildRenderObjectWidget {
   const CupertinoTextSelectionToolbar._({
-    Key key,
-    double barTopY,
-    double arrowTipX,
-    bool isArrowPointingDown,
-    Widget child,
+    Key? key,
+    required double barTopY,
+    required double arrowTipX,
+    required bool isArrowPointingDown,
+    Widget? child,
   }) : _barTopY = barTopY,
        _arrowTipX = arrowTipX,
        _isArrowPointingDown = isArrowPointingDown,
@@ -257,7 +255,7 @@ class _ToolbarParentData extends BoxParentData {
   // The x offset from the tip of the arrow to the center of the toolbar.
   // Positive if the tip of the arrow has a larger x-coordinate than the
   // center of the toolbar.
-  double arrowXOffsetFromCenter;
+  double? arrowXOffsetFromCenter;
   @override
   String toString() => 'offset=$offset, arrowXOffsetFromCenter=$arrowXOffsetFromCenter';
 }
@@ -267,7 +265,7 @@ class _ToolbarRenderBox extends RenderShiftedBox {
     this._barTopY,
     this._arrowTipX,
     this._isArrowPointingDown,
-    RenderBox child,
+    RenderBox? child,
   ) : super(child);
 
 
@@ -325,37 +323,37 @@ class _ToolbarRenderBox extends RenderShiftedBox {
       .deflate(const EdgeInsets.symmetric(horizontal: _kToolbarScreenPadding))
       .loosen();
 
-    child.layout(heightConstraint.enforce(enforcedConstraint), parentUsesSize: true,);
-    final _ToolbarParentData childParentData = child.parentData as _ToolbarParentData;
+    child!.layout(heightConstraint.enforce(enforcedConstraint), parentUsesSize: true,);
+    final _ToolbarParentData childParentData = child!.parentData as _ToolbarParentData;
 
     // The local x-coordinate of the center of the toolbar.
-    final double lowerBound = child.size.width/2 + _kToolbarScreenPadding;
-    final double upperBound = size.width - child.size.width/2 - _kToolbarScreenPadding;
-    final double adjustedCenterX = _arrowTipX.clamp(lowerBound, upperBound) as double;
+    final double lowerBound = child!.size.width/2 + _kToolbarScreenPadding;
+    final double upperBound = size.width - child!.size.width/2 - _kToolbarScreenPadding;
+    final double adjustedCenterX = _arrowTipX.clamp(lowerBound, upperBound);
 
-    childParentData.offset = Offset(adjustedCenterX - child.size.width / 2, _barTopY);
+    childParentData.offset = Offset(adjustedCenterX - child!.size.width / 2, _barTopY);
     childParentData.arrowXOffsetFromCenter = _arrowTipX - adjustedCenterX;
   }
 
   // The path is described in the toolbar's coordinate system.
   Path _clipPath() {
-    final _ToolbarParentData childParentData = child.parentData as _ToolbarParentData;
+    final _ToolbarParentData childParentData = child!.parentData as _ToolbarParentData;
     final Path rrect = Path()
       ..addRRect(
         RRect.fromRectAndRadius(
-          Offset(0, _isArrowPointingDown ? 0 : _kToolbarArrowSize.height,)
-          & Size(child.size.width, child.size.height - _kToolbarArrowSize.height),
+          Offset(0, _isArrowPointingDown ? 0 : _kToolbarArrowSize.height)
+            & Size(child!.size.width, child!.size.height - _kToolbarArrowSize.height),
           _kToolbarBorderRadius,
         ),
       );
 
-    final double arrowTipX = child.size.width / 2 + childParentData.arrowXOffsetFromCenter;
+    final double arrowTipX = child!.size.width / 2 + childParentData.arrowXOffsetFromCenter!;
 
     final double arrowBottomY = _isArrowPointingDown
-      ? child.size.height - _kToolbarArrowSize.height
+      ? child!.size.height - _kToolbarArrowSize.height
       : _kToolbarArrowSize.height;
 
-    final double arrowTipY = _isArrowPointingDown ? child.size.height : 0;
+    final double arrowTipY = _isArrowPointingDown ? child!.size.height : 0;
 
     final Path arrow = Path()
       ..moveTo(arrowTipX, arrowTipY)
@@ -372,17 +370,17 @@ class _ToolbarRenderBox extends RenderShiftedBox {
       return;
     }
 
-    final _ToolbarParentData childParentData = child.parentData as _ToolbarParentData;
+    final _ToolbarParentData childParentData = child!.parentData as _ToolbarParentData;
     context.pushClipPath(
       needsCompositing,
       offset + childParentData.offset,
-      Offset.zero & child.size,
+      Offset.zero & child!.size,
       _clipPath(),
-      (PaintingContext innerContext, Offset innerOffset) => innerContext.paintChild(child, innerOffset),
+      (PaintingContext innerContext, Offset innerOffset) => innerContext.paintChild(child!, innerOffset),
     );
   }
 
-  Paint _debugPaint;
+  Paint? _debugPaint;
 
   @override
   void debugPaintSize(PaintingContext context, Offset offset) {
@@ -395,15 +393,15 @@ class _ToolbarRenderBox extends RenderShiftedBox {
         ..shader = ui.Gradient.linear(
           const Offset(0.0, 0.0),
           const Offset(10.0, 10.0),
-        <Color>[const Color(0x00000000), const Color(0xFFFF00FF), const Color(0xFFFF00FF), const Color(0x00000000)],
-        <double>[0.25, 0.25, 0.75, 0.75],
+          const <Color>[Color(0x00000000), Color(0xFFFF00FF), Color(0xFFFF00FF), Color(0x00000000)],
+          const <double>[0.25, 0.25, 0.75, 0.75],
           TileMode.repeated,
         )
         ..strokeWidth = 2.0
         ..style = PaintingStyle.stroke;
 
-      final _ToolbarParentData childParentData = child.parentData as _ToolbarParentData;
-      context.canvas.drawPath(_clipPath().shift(offset + childParentData.offset), _debugPaint);
+      final _ToolbarParentData childParentData = child!.parentData as _ToolbarParentData;
+      context.canvas.drawPath(_clipPath().shift(offset + childParentData.offset), _debugPaint!);
       return true;
     }());
   }
@@ -463,7 +461,7 @@ class _CupertinoTextSelectionControls extends TextSelectionControls {
     ClipboardStatusNotifier clipboardStatus,
   ) {
     assert(debugCheckHasMediaQuery(context));
-    final MediaQueryData mediaQuery = MediaQuery.of(context);
+    final MediaQueryData mediaQuery = MediaQuery.of(context)!;
 
     // The toolbar should appear below the TextField when there is not enough
     // space above the TextField to show it, assuming there's always enough space
@@ -478,7 +476,7 @@ class _CupertinoTextSelectionControls extends TextSelectionControls {
     final double arrowTipX = (position.dx + globalEditableRegion.left).clamp(
       _kArrowScreenPadding + mediaQuery.padding.left,
       mediaQuery.size.width - mediaQuery.padding.right - _kArrowScreenPadding,
-    ) as double;
+    );
 
     // The y-coordinate has to be calculated instead of directly quoting position.dy,
     // since the caller (TextSelectionOverlay._buildToolbar) does not know whether
@@ -532,8 +530,6 @@ class _CupertinoTextSelectionControls extends TextSelectionControls {
       case TextSelectionHandleType.collapsed:
         return const SizedBox();
     }
-    assert(type != null);
-    return null;
   }
 
   /// Gets anchor for cupertino-style text selection handles.
@@ -570,9 +566,9 @@ class _CupertinoTextSelectionControls extends TextSelectionControls {
 // Renders the content of the selection menu and maintains the page state.
 class _CupertinoTextSelectionToolbarContent extends StatefulWidget {
   const _CupertinoTextSelectionToolbarContent({
-    Key key,
-    @required this.children,
-    @required this.isArrowPointingDown,
+    Key? key,
+    required this.children,
+    required this.isArrowPointingDown,
   }) : assert(children != null),
        // This ignore is used because .isNotEmpty isn't compatible with const.
        assert(children.length > 0), // ignore: prefer_is_empty
@@ -587,9 +583,9 @@ class _CupertinoTextSelectionToolbarContent extends StatefulWidget {
 
 class _CupertinoTextSelectionToolbarContentState extends State<_CupertinoTextSelectionToolbarContent> with TickerProviderStateMixin {
   // Controls the fading of the buttons within the menu during page transitions.
-  AnimationController _controller;
+  late AnimationController _controller;
   int _page = 0;
-  int _nextPage;
+  int? _nextPage;
 
   void _handleNextPage() {
     _controller.reverse();
@@ -609,7 +605,7 @@ class _CupertinoTextSelectionToolbarContentState extends State<_CupertinoTextSel
     }
 
     setState(() {
-      _page = _nextPage;
+      _page = _nextPage!;
       _nextPage = null;
     });
     _controller.forward();
@@ -666,7 +662,7 @@ class _CupertinoTextSelectionToolbarContentState extends State<_CupertinoTextSel
             pressedOpacity: 0.7,
             child: const Text('◀', style: _kToolbarButtonFontStyle),
           ),
-          dividerWidth: 1.0 / MediaQuery.of(context).devicePixelRatio,
+          dividerWidth: 1.0 / MediaQuery.of(context)!.devicePixelRatio,
           nextButton: CupertinoButton(
             borderRadius: null,
             color: _kToolbarBackgroundColor,
@@ -698,13 +694,13 @@ class _CupertinoTextSelectionToolbarContentState extends State<_CupertinoTextSel
 // _CupertinoTextSelectionToolbarItemsElement, paginates the menu items.
 class _CupertinoTextSelectionToolbarItems extends RenderObjectWidget {
   _CupertinoTextSelectionToolbarItems({
-    Key key,
-    @required this.page,
-    @required this.children,
-    @required this.backButton,
-    @required this.dividerWidth,
-    @required this.nextButton,
-    @required this.nextButtonDisabled,
+    Key? key,
+    required this.page,
+    required this.children,
+    required this.backButton,
+    required this.dividerWidth,
+    required this.nextButton,
+    required this.nextButtonDisabled,
   }) : assert(children != null),
        assert(children.isNotEmpty),
        assert(backButton != null),
@@ -746,7 +742,7 @@ class _CupertinoTextSelectionToolbarItemsElement extends RenderObjectElement {
     _CupertinoTextSelectionToolbarItems widget,
   ) : super(widget);
 
-  List<Element> _children;
+  late List<Element> _children;
   final Map<_CupertinoTextSelectionToolbarItemsSlot, Element> slotToChild = <_CupertinoTextSelectionToolbarItemsSlot, Element>{};
 
   // We keep a set of forgotten children to avoid O(n^2) work walking _children
@@ -759,7 +755,7 @@ class _CupertinoTextSelectionToolbarItemsElement extends RenderObjectElement {
   @override
   _CupertinoTextSelectionToolbarItemsRenderBox get renderObject => super.renderObject as _CupertinoTextSelectionToolbarItemsRenderBox;
 
-  void _updateRenderObject(RenderBox child, _CupertinoTextSelectionToolbarItemsSlot slot) {
+  void _updateRenderObject(RenderBox? child, _CupertinoTextSelectionToolbarItemsSlot slot) {
     switch (slot) {
       case _CupertinoTextSelectionToolbarItemsSlot.backButton:
         renderObject.backButton = child;
@@ -783,7 +779,7 @@ class _CupertinoTextSelectionToolbarItemsElement extends RenderObjectElement {
     }
     if (slot is IndexedSlot) {
       assert(renderObject.debugValidateChild(child));
-      renderObject.insert(child as RenderBox, after: slot?.value?.renderObject as RenderBox);
+      renderObject.insert(child as RenderBox, after: slot.value?.renderObject as RenderBox);
       return;
     }
     assert(false, 'slot must be _CupertinoTextSelectionToolbarItemsSlot or IndexedSlot');
@@ -793,11 +789,11 @@ class _CupertinoTextSelectionToolbarItemsElement extends RenderObjectElement {
   @override
   void moveRenderObjectChild(RenderObject child, IndexedSlot<Element> oldSlot, IndexedSlot<Element> newSlot) {
     assert(child.parent == renderObject);
-    renderObject.move(child as RenderBox, after: newSlot?.value?.renderObject as RenderBox);
+    renderObject.move(child as RenderBox, after: newSlot.value.renderObject as RenderBox);
   }
 
   static bool _shouldPaint(Element child) {
-    return (child.renderObject.parentData as ToolbarItemsParentData).shouldPaint;
+    return (child.renderObject!.parentData as ToolbarItemsParentData).shouldPaint;
   }
 
   @override
@@ -842,8 +838,8 @@ class _CupertinoTextSelectionToolbarItemsElement extends RenderObjectElement {
 
   // Mount or update slotted child.
   void _mountChild(Widget widget, _CupertinoTextSelectionToolbarItemsSlot slot) {
-    final Element oldChild = slotToChild[slot];
-    final Element newChild = updateChild(oldChild, widget, slot);
+    final Element? oldChild = slotToChild[slot];
+    final Element? newChild = updateChild(oldChild, widget, slot);
     if (oldChild != null) {
       slotToChild.remove(slot);
     }
@@ -853,7 +849,7 @@ class _CupertinoTextSelectionToolbarItemsElement extends RenderObjectElement {
   }
 
   @override
-  void mount(Element parent, dynamic newSlot) {
+  void mount(Element? parent, dynamic newSlot) {
     super.mount(parent, newSlot);
     // Mount slotted children.
     _mountChild(widget.backButton, _CupertinoTextSelectionToolbarItemsSlot.backButton);
@@ -861,10 +857,10 @@ class _CupertinoTextSelectionToolbarItemsElement extends RenderObjectElement {
     _mountChild(widget.nextButtonDisabled, _CupertinoTextSelectionToolbarItemsSlot.nextButtonDisabled);
 
     // Mount list children.
-    _children = List<Element>(widget.children.length);
-    Element previousChild;
+    _children = List<Element>.filled(widget.children.length, _NullElement.instance);
+    Element? previousChild;
     for (int i = 0; i < _children.length; i += 1) {
-      final Element newChild = inflateWidget(widget.children[i], IndexedSlot<Element>(i, previousChild));
+      final Element newChild = inflateWidget(widget.children[i], IndexedSlot<Element?>(i, previousChild));
       _children[i] = newChild;
       previousChild = newChild;
     }
@@ -903,8 +899,8 @@ class _CupertinoTextSelectionToolbarItemsElement extends RenderObjectElement {
 // The custom RenderBox that helps paginate the menu items.
 class _CupertinoTextSelectionToolbarItemsRenderBox extends RenderBox with ContainerRenderObjectMixin<RenderBox, ToolbarItemsParentData>, RenderBoxContainerDefaultsMixin<RenderBox, ToolbarItemsParentData> {
   _CupertinoTextSelectionToolbarItemsRenderBox({
-    @required double dividerWidth,
-    @required int page,
+    required double dividerWidth,
+    required int page,
   }) : assert(dividerWidth != null),
        assert(page != null),
        _dividerWidth = dividerWidth,
@@ -913,7 +909,7 @@ class _CupertinoTextSelectionToolbarItemsRenderBox extends RenderBox with Contai
 
   final Map<_CupertinoTextSelectionToolbarItemsSlot, RenderBox> slottedChildren = <_CupertinoTextSelectionToolbarItemsSlot, RenderBox>{};
 
-  RenderBox _updateChild(RenderBox oldChild, RenderBox newChild, _CupertinoTextSelectionToolbarItemsSlot slot) {
+  RenderBox? _updateChild(RenderBox? oldChild, RenderBox? newChild, _CupertinoTextSelectionToolbarItemsSlot slot) {
     if (oldChild != null) {
       dropChild(oldChild);
       slottedChildren.remove(slot);
@@ -949,21 +945,21 @@ class _CupertinoTextSelectionToolbarItemsRenderBox extends RenderBox with Contai
     markNeedsLayout();
   }
 
-  RenderBox _backButton;
-  RenderBox get backButton => _backButton;
-  set backButton(RenderBox value) {
+  RenderBox? _backButton;
+  RenderBox? get backButton => _backButton;
+  set backButton(RenderBox? value) {
     _backButton = _updateChild(_backButton, value, _CupertinoTextSelectionToolbarItemsSlot.backButton);
   }
 
-  RenderBox _nextButton;
-  RenderBox get nextButton => _nextButton;
-  set nextButton(RenderBox value) {
+  RenderBox? _nextButton;
+  RenderBox? get nextButton => _nextButton;
+  set nextButton(RenderBox? value) {
     _nextButton = _updateChild(_nextButton, value, _CupertinoTextSelectionToolbarItemsSlot.nextButton);
   }
 
-  RenderBox _nextButtonDisabled;
-  RenderBox get nextButtonDisabled => _nextButtonDisabled;
-  set nextButtonDisabled(RenderBox value) {
+  RenderBox? _nextButtonDisabled;
+  RenderBox? get nextButtonDisabled => _nextButtonDisabled;
+  set nextButtonDisabled(RenderBox? value) {
     _nextButtonDisabled = _updateChild(_nextButtonDisabled, value, _CupertinoTextSelectionToolbarItemsSlot.nextButtonDisabled);
   }
 
@@ -975,15 +971,15 @@ class _CupertinoTextSelectionToolbarItemsRenderBox extends RenderBox with Contai
     }
 
     // Layout slotted children.
-    _backButton.layout(constraints.loosen(), parentUsesSize: true);
-    _nextButton.layout(constraints.loosen(), parentUsesSize: true);
-    _nextButtonDisabled.layout(constraints.loosen(), parentUsesSize: true);
+    _backButton!.layout(constraints.loosen(), parentUsesSize: true);
+    _nextButton!.layout(constraints.loosen(), parentUsesSize: true);
+    _nextButtonDisabled!.layout(constraints.loosen(), parentUsesSize: true);
 
     final double subsequentPageButtonsWidth =
-        _backButton.size.width + _nextButton.size.width;
+        _backButton!.size.width + _nextButton!.size.width;
     double currentButtonPosition = 0.0;
-    double toolbarWidth; // The width of the whole widget.
-    double firstPageWidth;
+    late double toolbarWidth; // The width of the whole widget.
+    late double firstPageWidth;
     int currentPage = 0;
     int i = -1;
     visitChildren((RenderObject renderObjectChild) {
@@ -1002,7 +998,7 @@ class _CupertinoTextSelectionToolbarItemsRenderBox extends RenderBox with Contai
       if (currentPage == 0) {
         // If this is the last child, it's ok to fit without a forward button.
         paginationButtonsWidth =
-            i == childCount - 1 ? 0.0 : _nextButton.size.width;
+            i == childCount - 1 ? 0.0 : _nextButton!.size.width;
       } else {
         paginationButtonsWidth = subsequentPageButtonsWidth;
       }
@@ -1022,8 +1018,8 @@ class _CupertinoTextSelectionToolbarItemsRenderBox extends RenderBox with Contai
           currentButtonPosition + paginationButtonsWidth + child.size.width;
       if (currentWidth > constraints.maxWidth) {
         currentPage++;
-        currentButtonPosition = _backButton.size.width + dividerWidth;
-        paginationButtonsWidth = _backButton.size.width + _nextButton.size.width;
+        currentButtonPosition = _backButton!.size.width + dividerWidth;
+        paginationButtonsWidth = _backButton!.size.width + _nextButton!.size.width;
         child.layout(
           BoxConstraints.loose(Size(
             firstPageWidth - paginationButtonsWidth,
@@ -1037,7 +1033,7 @@ class _CupertinoTextSelectionToolbarItemsRenderBox extends RenderBox with Contai
       childParentData.shouldPaint = currentPage == page;
 
       if (currentPage == 0) {
-        firstPageWidth = currentButtonPosition + _nextButton.size.width;
+        firstPageWidth = currentButtonPosition + _nextButton!.size.width;
       }
       if (currentPage == page) {
         toolbarWidth = currentButtonPosition;
@@ -1050,21 +1046,21 @@ class _CupertinoTextSelectionToolbarItemsRenderBox extends RenderBox with Contai
     // Position page nav buttons.
     if (currentPage > 0) {
       final ToolbarItemsParentData nextButtonParentData =
-          _nextButton.parentData as ToolbarItemsParentData;
+          _nextButton!.parentData as ToolbarItemsParentData;
       final ToolbarItemsParentData nextButtonDisabledParentData =
-          _nextButtonDisabled.parentData as ToolbarItemsParentData;
+          _nextButtonDisabled!.parentData as ToolbarItemsParentData;
       final ToolbarItemsParentData backButtonParentData =
-          _backButton.parentData as ToolbarItemsParentData;
+          _backButton!.parentData as ToolbarItemsParentData;
       // The forward button always shows if there is more than one page, even on
       // the last page (it's just disabled).
       if (page == currentPage) {
         nextButtonDisabledParentData.offset = Offset(toolbarWidth, 0.0);
         nextButtonDisabledParentData.shouldPaint = true;
-        toolbarWidth += nextButtonDisabled.size.width;
+        toolbarWidth += nextButtonDisabled!.size.width;
       } else {
         nextButtonParentData.offset = Offset(toolbarWidth, 0.0);
         nextButtonParentData.shouldPaint = true;
-        toolbarWidth += nextButton.size.width;
+        toolbarWidth += nextButton!.size.width;
       }
       if (page > 0) {
         backButtonParentData.offset = Offset.zero;
@@ -1102,7 +1098,7 @@ class _CupertinoTextSelectionToolbarItemsRenderBox extends RenderBox with Contai
   }
 
   // Returns true iff the single child is hit by the given position.
-  static bool hitTestChild(RenderBox child, BoxHitTestResult result, { Offset position }) {
+  static bool hitTestChild(RenderBox? child, BoxHitTestResult result, { required Offset position }) {
     if (child == null) {
       return false;
     }
@@ -1119,10 +1115,10 @@ class _CupertinoTextSelectionToolbarItemsRenderBox extends RenderBox with Contai
   }
 
   @override
-  bool hitTestChildren(BoxHitTestResult result, { Offset position }) {
+  bool hitTestChildren(BoxHitTestResult result, { required Offset position }) {
     // Hit test list children.
     // The x, y parameters have the top left of the node's box as the origin.
-    RenderBox child = lastChild;
+    RenderBox? child = lastChild;
     while (child != null) {
       final ToolbarItemsParentData childParentData = child.parentData as ToolbarItemsParentData;
 
@@ -1186,13 +1182,13 @@ class _CupertinoTextSelectionToolbarItemsRenderBox extends RenderBox with Contai
   void visitChildren(RenderObjectVisitor visitor) {
     // Visit the slotted children.
     if (_backButton != null) {
-      visitor(_backButton);
+      visitor(_backButton!);
     }
     if (_nextButton != null) {
-      visitor(_nextButton);
+      visitor(_nextButton!);
     }
     if (_nextButtonDisabled != null) {
-      visitor(_nextButtonDisabled);
+      visitor(_nextButtonDisabled!);
     }
     // Visit the list children.
     super.visitChildren(visitor);
@@ -1214,9 +1210,6 @@ class _CupertinoTextSelectionToolbarItemsRenderBox extends RenderBox with Contai
   List<DiagnosticsNode> debugDescribeChildren() {
     final List<DiagnosticsNode> value = <DiagnosticsNode>[];
     visitChildren((RenderObject renderObjectChild) {
-      if (renderObjectChild == null) {
-        return;
-      }
       final RenderBox child = renderObjectChild as RenderBox;
       if (child == backButton) {
         value.add(child.toDiagnosticsNode(name: 'back button'));
@@ -1244,3 +1237,20 @@ enum _CupertinoTextSelectionToolbarItemsSlot {
 
 /// Text selection controls that follows iOS design conventions.
 final TextSelectionControls cupertinoTextSelectionControls = _CupertinoTextSelectionControls();
+
+class _NullElement extends Element {
+  _NullElement() : super(_NullWidget());
+
+  static _NullElement instance = _NullElement();
+
+  @override
+  bool get debugDoingBuild => throw UnimplementedError();
+
+  @override
+  void performRebuild() { }
+}
+
+class _NullWidget extends Widget {
+  @override
+  Element createElement() => throw UnimplementedError();
+}
\ No newline at end of file