Make Material/CupertinoLocalizations non-nullable (#68807)

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

@@ -209,14 +209,7 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
 
   List<Widget> _buildTabItems(BuildContext context) {
     final List<Widget> result = <Widget>[];
-    final CupertinoLocalizations? localizations = CupertinoLocalizations.of(context);
+    final CupertinoLocalizations localizations = CupertinoLocalizations.of(context);
-    assert(
-      localizations != null,
-      'CupertinoTabBar requires a Localizations parent in order to provide an '
-        'appropriate Semantics hint for tab indexing. A CupertinoApp will '
-        'provide the DefaultCupertinoLocalizations, or you can instantiate your '
-        'own Localizations.'
-    );
 
     for (int index = 0; index < items.length; index += 1) {
       final bool active = index == currentIndex;
@@ -226,7 +219,7 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
           Expanded(
             child: Semantics(
               selected: active,
-              hint: localizations!.tabSemanticsLabel(
+              hint: localizations.tabSemanticsLabel(
                 tabIndex: index + 1,
                 tabCount: items.length,
               ),

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

@@ -623,7 +623,7 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
     super.didChangeDependencies();
 
     textDirectionFactor = Directionality.of(context) == TextDirection.ltr ? 1 : -1;
-    localizations = CupertinoLocalizations.of(context)!;
+    localizations = CupertinoLocalizations.of(context);
 
     alignCenterLeft = textDirectionFactor == 1 ? Alignment.centerLeft : Alignment.centerRight;
     alignCenterRight = textDirectionFactor == 1 ? Alignment.centerRight : Alignment.centerLeft;
@@ -1113,7 +1113,7 @@ class _CupertinoDatePickerDateState extends State<CupertinoDatePicker> {
     super.didChangeDependencies();
 
     textDirectionFactor = Directionality.of(context) == TextDirection.ltr ? 1 : -1;
-    localizations = CupertinoLocalizations.of(context)!;
+    localizations = CupertinoLocalizations.of(context);
 
     alignCenterLeft = textDirectionFactor == 1 ? Alignment.centerLeft : Alignment.centerRight;
     alignCenterRight = textDirectionFactor == 1 ? Alignment.centerRight : Alignment.centerLeft;
@@ -1621,7 +1621,7 @@ class _CupertinoTimerPickerState extends State<CupertinoTimerPicker> {
     super.didChangeDependencies();
 
     textDirection = Directionality.of(context)!;
-    localizations = CupertinoLocalizations.of(context)!;
+    localizations = CupertinoLocalizations.of(context);
 
     _measureLabelMetrics();
   }

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

@@ -0,0 +1,46 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter/widgets.dart';
+
+import 'localizations.dart';
+
+/// Asserts that the given context has a [Localizations] ancestor that contains
+/// a [CupertinoLocalizations] delegate.
+///
+/// To call this function, use the following pattern, typically in the
+/// relevant Widget's build method:
+///
+/// ```dart
+/// assert(debugCheckHasCupertinoLocalizations(context));
+/// ```
+///
+/// Does nothing if asserts are disabled. Always returns true.
+bool debugCheckHasCupertinoLocalizations(BuildContext context) {
+  assert(() {
+    if (Localizations.of<CupertinoLocalizations>(context, CupertinoLocalizations) == null) {
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('No CupertinoLocalizations found.'),
+        ErrorDescription(
+          '${context.widget.runtimeType} widgets require CupertinoLocalizations '
+          'to be provided by a Localizations widget ancestor.'
+        ),
+        ErrorDescription(
+          'The cupertino library uses Localizations to generate messages, '
+          'labels, and abbreviations.'
+        ),
+        ErrorHint(
+          'To introduce a CupertinoLocalizations, either use a '
+          'CupertinoApp at the root of your application to include them '
+          'automatically, or add a Localization widget with a '
+          'CupertinoLocalizations delegate.'
+        ),
+        ...context.describeMissingAncestor(expectedAncestorType: CupertinoLocalizations)
+      ]);
+    }
+    return true;
+  }());
+  return true;
+}

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

@@ -226,7 +226,7 @@ class CupertinoAlertDialog extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final CupertinoLocalizations localizations = CupertinoLocalizations.of(context)!;
+    final CupertinoLocalizations localizations = CupertinoLocalizations.of(context);
     final bool isInAccessibilityMode = _isInAccessibilityMode(context);
     final double textScaleFactor = MediaQuery.of(context)!.textScaleFactor;
     return CupertinoUserInterfaceLevel(

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

@@ -5,7 +5,7 @@
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 
-import 'date_picker.dart';
+import 'debug.dart';
 
 /// Determines the order of the columns inside [CupertinoDatePicker] in
 /// time and date time mode.
@@ -242,8 +242,11 @@ abstract class CupertinoLocalizations {
   /// The `CupertinoLocalizations` from the closest [Localizations] instance
   /// that encloses the given context.
   ///
+  /// If no [CupertinoLocalizations] are available in the given `context`, this
+  /// method throws an exception.
+  ///
   /// This method is just a convenient shorthand for:
-  /// `Localizations.of<CupertinoLocalizations>(context, CupertinoLocalizations)`.
+  /// `Localizations.of<CupertinoLocalizations>(context, CupertinoLocalizations)!`.
   ///
   /// References to the localized resources defined by this class are typically
   /// written in terms of this method. For example:
@@ -251,8 +254,9 @@ abstract class CupertinoLocalizations {
   /// ```dart
   /// CupertinoLocalizations.of(context).anteMeridiemAbbreviation;
   /// ```
-  static CupertinoLocalizations? of(BuildContext context) {
+  static CupertinoLocalizations of(BuildContext context) {
-    return Localizations.of<CupertinoLocalizations>(context, CupertinoLocalizations);
+    debugCheckHasCupertinoLocalizations(context);
+    return Localizations.of<CupertinoLocalizations>(context, CupertinoLocalizations)!;
   }
 }
 

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

@@ -1146,7 +1146,7 @@ Future<T?> showCupertinoDialog<T>({
   return showGeneralDialog(
     context: context,
     barrierDismissible: barrierDismissible,
-    barrierLabel: CupertinoLocalizations.of(context)!.modalBarrierDismissLabel,
+    barrierLabel: CupertinoLocalizations.of(context).modalBarrierDismissLabel,
     barrierColor: CupertinoDynamicColor.resolve(_kModalBarrierColor, context)!,
     // This transition duration was eyeballed comparing with iOS
     transitionDuration: const Duration(milliseconds: 250),

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

@@ -152,7 +152,7 @@ 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);
@@ -183,17 +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._(

packages/flutter/lib/src/material/about.dart

@@ -201,7 +201,7 @@ class AboutListTile extends StatelessWidget {
     assert(debugCheckHasMaterialLocalizations(context));
     return ListTile(
       leading: icon,
-      title: child ?? Text(MaterialLocalizations.of(context)!.aboutListTileTitle(
+      title: child ?? Text(MaterialLocalizations.of(context).aboutListTileTitle(
         applicationName ?? _defaultApplicationName(context),
       )),
       dense: dense,
@@ -405,7 +405,7 @@ class AboutDialog extends StatelessWidget {
       ),
       actions: <Widget>[
         TextButton(
-          child: Text(MaterialLocalizations.of(context)!.viewLicensesButtonLabel),
+          child: Text(MaterialLocalizations.of(context).viewLicensesButtonLabel),
           onPressed: () {
             showLicensePage(
               context: context,
@@ -417,7 +417,7 @@ class AboutDialog extends StatelessWidget {
           },
         ),
         TextButton(
-          child: Text(MaterialLocalizations.of(context)!.closeButtonLabel),
+          child: Text(MaterialLocalizations.of(context).closeButtonLabel),
           onPressed: () {
             Navigator.pop(context);
           },
@@ -493,7 +493,7 @@ class _LicensePageState extends State<LicensePage> {
   Widget build(BuildContext context) {
     return _MasterDetailFlow(
       detailPageFABlessGutterWidth: _getGutterSize(context),
-      title: Text(MaterialLocalizations.of(context)!.licensesPageTitle),
+      title: Text(MaterialLocalizations.of(context).licensesPageTitle),
       detailPageBuilder: _packageLicensePage,
       masterViewBuilder: _packagesView,
     );
@@ -720,7 +720,7 @@ class _PackageListTile extends StatelessWidget {
       color: isSelected ? Theme.of(context)!.highlightColor : Theme.of(context)!.cardColor,
       child: ListTile(
         title: Text(packageName),
-        subtitle: Text(MaterialLocalizations.of(context)!.licensesPackageDetailText(numberLicenses)),
+        subtitle: Text(MaterialLocalizations.of(context).licensesPackageDetailText(numberLicenses)),
         selected: isSelected,
         onTap: onTap,
       ),
@@ -890,7 +890,7 @@ class _PackageLicensePageState extends State<_PackageLicensePage> {
   @override
   Widget build(BuildContext context) {
     assert(debugCheckHasMaterialLocalizations(context));
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final ThemeData? theme = Theme.of(context);
     final String title = widget.packageName;
     final String subtitle = localizations.licensesPackageDetailText(widget.licenseEntries.length);

packages/flutter/lib/src/material/app_bar.dart

@@ -543,7 +543,7 @@ class _AppBarState extends State<AppBar> {
         leading = IconButton(
           icon: const Icon(Icons.menu),
           onPressed: _handleDrawerButton,
-          tooltip: MaterialLocalizations.of(context)!.openAppDrawerTooltip,
+          tooltip: MaterialLocalizations.of(context).openAppDrawerTooltip,
         );
       } else {
         if (!hasEndDrawer && canPop)
@@ -616,7 +616,7 @@ class _AppBarState extends State<AppBar> {
       actions = IconButton(
         icon: const Icon(Icons.menu),
         onPressed: _handleDrawerButtonEnd,
-        tooltip: MaterialLocalizations.of(context)!.openAppDrawerTooltip,
+        tooltip: MaterialLocalizations.of(context).openAppDrawerTooltip,
       );
     }
 

packages/flutter/lib/src/material/back_button.dart

@@ -98,7 +98,7 @@ class BackButton extends StatelessWidget {
     return IconButton(
       icon: const BackButtonIcon(),
       color: color,
-      tooltip: MaterialLocalizations.of(context)!.backButtonTooltip,
+      tooltip: MaterialLocalizations.of(context).backButtonTooltip,
       onPressed: () {
         if (onPressed != null) {
           onPressed!();
@@ -152,7 +152,7 @@ class CloseButton extends StatelessWidget {
     return IconButton(
       icon: const Icon(Icons.close),
       color: color,
-      tooltip: MaterialLocalizations.of(context)!.closeButtonTooltip,
+      tooltip: MaterialLocalizations.of(context).closeButtonTooltip,
       onPressed: () {
         if (onPressed != null) {
           onPressed!();

packages/flutter/lib/src/material/bottom_navigation_bar.dart

@@ -823,7 +823,7 @@ class _BottomNavigationBarState extends State<BottomNavigationBar> with TickerPr
   }
 
   List<Widget> _createTiles() {
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     assert(localizations != null);
 
     final ThemeData themeData = Theme.of(context)!;

packages/flutter/lib/src/material/bottom_sheet.dart

@@ -370,7 +370,7 @@ class _ModalBottomSheetState<T> extends State<_ModalBottomSheet<T>> {
     assert(debugCheckHasMediaQuery(context));
     assert(debugCheckHasMaterialLocalizations(context));
     final MediaQueryData? mediaQuery = MediaQuery.of(context);
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final String routeLabel = _getRouteLabel(localizations);
 
     return AnimatedBuilder(
@@ -671,7 +671,7 @@ Future<T?> showModalBottomSheet<T>({
     builder: builder,
     theme: Theme.of(context, shadowThemeOnly: true),
     isScrollControlled: isScrollControlled,
-    barrierLabel: MaterialLocalizations.of(context)!.modalBarrierDismissLabel,
+    barrierLabel: MaterialLocalizations.of(context).modalBarrierDismissLabel,
     backgroundColor: backgroundColor,
     elevation: elevation,
     shape: shape,

packages/flutter/lib/src/material/chip.dart

@@ -1789,7 +1789,7 @@ class _RawChipState extends State<RawChip> with TickerProviderStateMixin<RawChip
       container: true,
       button: true,
       child: _wrapWithTooltip(
-        widget.deleteButtonTooltipMessage ?? MaterialLocalizations.of(context)?.deleteButtonTooltip,
+        widget.deleteButtonTooltipMessage ?? MaterialLocalizations.of(context).deleteButtonTooltip,
         widget.onDeleted,
         GestureDetector(
           key: deleteIconKey,

packages/flutter/lib/src/material/dialog.dart

@@ -463,7 +463,7 @@ class AlertDialog extends StatelessWidget {
       case TargetPlatform.fuchsia:
       case TargetPlatform.linux:
       case TargetPlatform.windows:
-        label ??= MaterialLocalizations.of(context)?.alertDialogLabel;
+        label ??= MaterialLocalizations.of(context).alertDialogLabel;
     }
 
     // The paddingScaleFactor is used to adjust the padding of Dialog's
@@ -814,13 +814,12 @@ class SimpleDialog extends StatelessWidget {
       switch (theme.platform) {
         case TargetPlatform.macOS:
         case TargetPlatform.iOS:
-          label = semanticLabel;
           break;
         case TargetPlatform.android:
         case TargetPlatform.fuchsia:
         case TargetPlatform.linux:
         case TargetPlatform.windows:
-          label = semanticLabel ?? MaterialLocalizations.of(context)?.dialogLabel;
+          label = semanticLabel ?? MaterialLocalizations.of(context).dialogLabel;
       }
     }
 
@@ -997,7 +996,7 @@ Future<T?> showDialog<T>({
       return dialog;
     },
     barrierDismissible: barrierDismissible,
-    barrierLabel: MaterialLocalizations.of(context)!.modalBarrierDismissLabel,
+    barrierLabel: MaterialLocalizations.of(context).modalBarrierDismissLabel,
     barrierColor: barrierColor ?? Colors.black54,
     transitionDuration: const Duration(milliseconds: 150),
     transitionBuilder: _buildMaterialDialogTransitions,

packages/flutter/lib/src/material/drawer.dart

@@ -180,13 +180,12 @@ class Drawer extends StatelessWidget {
     switch (Theme.of(context)!.platform) {
       case TargetPlatform.iOS:
       case TargetPlatform.macOS:
-        label = semanticLabel;
         break;
       case TargetPlatform.android:
       case TargetPlatform.fuchsia:
       case TargetPlatform.linux:
       case TargetPlatform.windows:
-        label = semanticLabel ?? MaterialLocalizations.of(context)?.drawerLabel;
+        label = semanticLabel ?? MaterialLocalizations.of(context).drawerLabel;
     }
     return Semantics(
       scopesRoute: true,
@@ -566,7 +565,7 @@ class DrawerControllerState extends State<DrawerController> with SingleTickerPro
                   child: GestureDetector(
                     onTap: close,
                     child: Semantics(
-                      label: MaterialLocalizations.of(context)?.modalBarrierDismissLabel,
+                      label: MaterialLocalizations.of(context).modalBarrierDismissLabel,
                       child: MouseRegion(
                         opaque: true,
                         child: Container( // The drawer's "scrim"

packages/flutter/lib/src/material/dropdown.dart

@@ -253,7 +253,7 @@ class _DropdownMenuState<T> extends State<_DropdownMenu<T>> {
     // When the menu is dismissed we just fade the entire thing out
     // in the first 0.25s.
     assert(debugCheckHasMaterialLocalizations(context));
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final _DropdownRoute<T> route = widget.route;
     final List<Widget> children = <Widget>[
       for (int itemIndex = 0; itemIndex < route.items.length; ++itemIndex)
@@ -1216,7 +1216,7 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
       elevation: widget.elevation,
       theme: Theme.of(context, shadowThemeOnly: true),
       style: _textStyle!,
-      barrierLabel: MaterialLocalizations.of(context)!.modalBarrierDismissLabel,
+      barrierLabel: MaterialLocalizations.of(context).modalBarrierDismissLabel,
       itemHeight: widget.itemHeight,
       dropdownColor: widget.dropdownColor,
     );

packages/flutter/lib/src/material/expand_icon.dart

@@ -167,7 +167,7 @@ class _ExpandIconState extends State<ExpandIcon> with SingleTickerProviderStateM
   Widget build(BuildContext context) {
     assert(debugCheckHasMaterial(context));
     assert(debugCheckHasMaterialLocalizations(context));
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final String onTapHint = widget.isExpanded ? localizations.expandedIconTapHint : localizations.collapsedIconTapHint;
 
     return Semantics(

packages/flutter/lib/src/material/expansion_panel.dart

@@ -496,7 +496,7 @@ class _ExpansionPanelListState extends State<ExpansionPanelList> {
         ),
       );
       if (!child.canTapOnHeader) {
-        final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+        final MaterialLocalizations localizations = MaterialLocalizations.of(context);
         expandIconContainer = Semantics(
           label: _isChildExpanded(index)? localizations.expandedIconTapHint : localizations.collapsedIconTapHint,
           container: true,

packages/flutter/lib/src/material/material_localizations.dart

@@ -5,8 +5,7 @@
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 
-import 'reorderable_list.dart';
+import 'debug.dart';
-import 'text_theme.dart';
 import 'time.dart';
 import 'typography.dart';
 
@@ -494,8 +493,11 @@ abstract class MaterialLocalizations {
   /// The `MaterialLocalizations` from the closest [Localizations] instance
   /// that encloses the given context.
   ///
+  /// If no [MaterialLocalizations] are available in the given `context`, this
+  /// method throws an exception.
+  ///
   /// This method is just a convenient shorthand for:
-  /// `Localizations.of<MaterialLocalizations>(context, MaterialLocalizations)`.
+  /// `Localizations.of<MaterialLocalizations>(context, MaterialLocalizations)!`.
   ///
   /// References to the localized resources defined by this class are typically
   /// written in terms of this method. For example:
@@ -503,8 +505,9 @@ abstract class MaterialLocalizations {
   /// ```dart
   /// tooltip: MaterialLocalizations.of(context).backButtonTooltip,
   /// ```
-  static MaterialLocalizations? of(BuildContext context) {
+  static MaterialLocalizations of(BuildContext context) {
-    return Localizations.of<MaterialLocalizations>(context, MaterialLocalizations);
+    debugCheckHasMaterialLocalizations(context);
+    return Localizations.of<MaterialLocalizations>(context, MaterialLocalizations)!;
   }
 }
 

packages/flutter/lib/src/material/navigation_rail.dart

@@ -433,7 +433,7 @@ class _NavigationRailState extends State<NavigationRail> with TickerProviderStat
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context)!;
     final NavigationRailThemeData navigationRailTheme = NavigationRailTheme.of(context);
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
 
     final Color backgroundColor = widget.backgroundColor ?? navigationRailTheme.backgroundColor ?? theme.colorScheme.surface;
     final double elevation = widget.elevation ?? navigationRailTheme.elevation ?? 0;

packages/flutter/lib/src/material/paginated_data_table.dart

@@ -333,7 +333,7 @@ class PaginatedDataTableState extends State<PaginatedDataTable> {
     // TODO(ianh): This whole build function doesn't handle RTL yet.
     assert(debugCheckHasMaterialLocalizations(context));
     final ThemeData themeData = Theme.of(context)!;
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     // HEADER
     final List<Widget> headerWidgets = <Widget>[];
     double startPadding = 24.0;

packages/flutter/lib/src/material/pickers/calendar_date_picker.dart

@@ -179,7 +179,7 @@ class _CalendarDatePickerState extends State<CalendarDatePicker> {
     assert(debugCheckHasMaterial(context));
     assert(debugCheckHasMaterialLocalizations(context));
     assert(debugCheckHasDirectionality(context));
-    _localizations = MaterialLocalizations.of(context)!;
+    _localizations = MaterialLocalizations.of(context);
     _textDirection = Directionality.of(context)!;
     if (!_announcedInitialDate) {
       _announcedInitialDate = true;
@@ -380,7 +380,7 @@ class _DatePickerModeToggleButtonState extends State<_DatePickerModeToggleButton
         children: <Widget>[
           Flexible(
             child: Semantics(
-              label: MaterialLocalizations.of(context)!.selectYearSemanticsLabel,
+              label: MaterialLocalizations.of(context).selectYearSemanticsLabel,
               excludeSemantics: true,
               button: true,
               child: Container(
@@ -524,7 +524,7 @@ class _MonthPickerState extends State<_MonthPicker> {
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
-    _localizations = MaterialLocalizations.of(context)!;
+    _localizations = MaterialLocalizations.of(context);
     _textDirection = Directionality.of(context);
   }
 
@@ -941,7 +941,7 @@ class _DayPickerState extends State<_DayPicker> {
   @override
   Widget build(BuildContext context) {
     final ColorScheme colorScheme = Theme.of(context)!.colorScheme;
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final TextTheme textTheme = Theme.of(context)!.textTheme;
     final TextStyle? headerStyle = textTheme.caption?.apply(
       color: colorScheme.onSurface.withOpacity(0.60),

packages/flutter/lib/src/material/pickers/calendar_date_range_picker.dart

@@ -427,7 +427,7 @@ class _DayHeaders extends StatelessWidget {
     final ThemeData themeData = Theme.of(context)!;
     final ColorScheme colorScheme = themeData.colorScheme;
     final TextStyle textStyle = themeData.textTheme.subtitle2!.apply(color: colorScheme.onSurface);
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final List<Widget> labels = _getDayHeaders(textStyle, localizations);
 
     // Add leading and trailing containers for edges of the custom grid layout.
@@ -706,7 +706,7 @@ class _MonthItemState extends State<_MonthItem> {
     final ThemeData theme = Theme.of(context)!;
     final ColorScheme colorScheme = theme.colorScheme;
     final TextTheme textTheme = theme.textTheme;
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final TextDirection? textDirection = Directionality.of(context);
     final Color highlightColor = _highlightColor(context);
     final int day = dayToBuild.day;
@@ -818,7 +818,7 @@ class _MonthItemState extends State<_MonthItem> {
   Widget build(BuildContext context) {
     final ThemeData themeData = Theme.of(context)!;
     final TextTheme textTheme = themeData.textTheme;
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final int year = widget.displayedMonth.year;
     final int month = widget.displayedMonth.month;
     final int daysInMonth = utils.getDaysInMonth(year, month);

packages/flutter/lib/src/material/pickers/date_picker_deprecated.dart

@@ -239,7 +239,7 @@ class DayPicker extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final ThemeData themeData = Theme.of(context)!;
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final int year = displayedMonth.year;
     final int month = displayedMonth.month;
     final int daysInMonth = getDaysInMonth(year, month);

packages/flutter/lib/src/material/pickers/date_picker_dialog.dart

@@ -359,7 +359,7 @@ class _DatePickerDialogState extends State<_DatePickerDialog> {
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context)!;
     final ColorScheme colorScheme = theme.colorScheme;
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final Orientation orientation = MediaQuery.of(context)!.orientation;
     final TextTheme textTheme = theme.textTheme;
     // Constrain the textScaleFactor to the largest supported value to prevent

packages/flutter/lib/src/material/pickers/date_range_picker_dialog.dart

@@ -338,7 +338,7 @@ class _DateRangePickerDialogState extends State<_DateRangePickerDialog> {
     final MediaQueryData mediaQuery = MediaQuery.of(context)!;
     final Orientation orientation = mediaQuery.orientation;
     final double textScaleFactor = math.min(mediaQuery.textScaleFactor, 1.3);
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
 
     final Widget contents;
     final Size size;
@@ -478,7 +478,7 @@ class _CalendarRangePickerDialog extends StatelessWidget {
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context)!;
     final ColorScheme colorScheme = theme.colorScheme;
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final Orientation orientation = MediaQuery.of(context)!.orientation;
     final TextTheme textTheme = theme.textTheme;
     final Color headerForeground = colorScheme.brightness == Brightness.light
@@ -615,7 +615,7 @@ class _InputDateRangePickerDialog extends StatelessWidget {
   final String? helpText;
 
   String _formatDateRange(BuildContext context, DateTime? start, DateTime? end, DateTime now) {
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final String startText = utils.formatRangeStartDate(localizations, start, end);
     final String endText = utils.formatRangeEndDate(localizations, start, end, now);
     if (start == null || end == null) {
@@ -632,7 +632,7 @@ class _InputDateRangePickerDialog extends StatelessWidget {
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context)!;
     final ColorScheme colorScheme = theme.colorScheme;
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final Orientation orientation = MediaQuery.of(context)!.orientation;
     final TextTheme textTheme = theme.textTheme;
 

packages/flutter/lib/src/material/pickers/input_date_picker.dart

@@ -157,7 +157,7 @@ class _InputDatePickerFormFieldState extends State<InputDatePickerFormField> {
   void didChangeDependencies() {
     super.didChangeDependencies();
     if (_selectedDate != null) {
-      final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+      final MaterialLocalizations localizations = MaterialLocalizations.of(context);
       _inputText = localizations.formatCompactDate(_selectedDate!);
       TextEditingValue textEditingValue = _controller.value.copyWith(text: _inputText);
       // Select the new text if we are auto focused and haven't selected the text before.
@@ -173,7 +173,7 @@ class _InputDatePickerFormFieldState extends State<InputDatePickerFormField> {
   }
 
   DateTime? _parseDate(String? text) {
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     return localizations.parseCompactDate(text);
   }
 
@@ -188,9 +188,9 @@ class _InputDatePickerFormFieldState extends State<InputDatePickerFormField> {
   String? _validateDate(String? text) {
     final DateTime? date = _parseDate(text);
     if (date == null) {
-      return widget.errorFormatText ?? MaterialLocalizations.of(context)!.invalidDateFormatLabel;
+      return widget.errorFormatText ?? MaterialLocalizations.of(context).invalidDateFormatLabel;
     } else if (!_isValidAcceptableDate(date)) {
-      return widget.errorInvalidText ?? MaterialLocalizations.of(context)!.dateOutOfRangeLabel;
+      return widget.errorInvalidText ?? MaterialLocalizations.of(context).dateOutOfRangeLabel;
     }
     return null;
   }
@@ -219,7 +219,7 @@ class _InputDatePickerFormFieldState extends State<InputDatePickerFormField> {
 
   @override
   Widget build(BuildContext context) {
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final InputDecorationTheme inputTheme = Theme.of(context)!.inputDecorationTheme;
     return TextFormField(
       decoration: InputDecoration(

packages/flutter/lib/src/material/pickers/input_date_range_picker.dart

@@ -141,7 +141,7 @@ class InputDateRangePickerState extends State<InputDateRangePicker> {
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     if (_startDate != null) {
       _startInputText = localizations.formatCompactDate(_startDate!);
       final bool selectText = widget.autofocus && !_autoSelected;
@@ -166,7 +166,7 @@ class InputDateRangePickerState extends State<InputDateRangePicker> {
     final String? endError = _validateDate(_endDate);
     if (startError == null && endError == null) {
       if (_startDate!.isAfter(_endDate!)) {
-        startError = widget.errorInvalidRangeText ?? MaterialLocalizations.of(context)!.invalidDateRangeLabel;
+        startError = widget.errorInvalidRangeText ?? MaterialLocalizations.of(context).invalidDateRangeLabel;
       }
     }
     setState(() {
@@ -177,15 +177,15 @@ class InputDateRangePickerState extends State<InputDateRangePicker> {
   }
 
   DateTime? _parseDate(String? text) {
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     return localizations.parseCompactDate(text);
   }
 
   String? _validateDate(DateTime? date) {
     if (date == null) {
-      return widget.errorFormatText ?? MaterialLocalizations.of(context)!.invalidDateFormatLabel;
+      return widget.errorFormatText ?? MaterialLocalizations.of(context).invalidDateFormatLabel;
     } else if (date.isBefore(widget.firstDate) || date.isAfter(widget.lastDate)) {
-      return widget.errorInvalidText ?? MaterialLocalizations.of(context)!.dateOutOfRangeLabel;
+      return widget.errorInvalidText ?? MaterialLocalizations.of(context).dateOutOfRangeLabel;
     }
     return null;
   }
@@ -225,7 +225,7 @@ class InputDateRangePickerState extends State<InputDateRangePicker> {
 
   @override
   Widget build(BuildContext context) {
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final InputDecorationTheme inputTheme = Theme.of(context)!.inputDecorationTheme;
     return Row(
       crossAxisAlignment: CrossAxisAlignment.start,

packages/flutter/lib/src/material/popup_menu.dart

@@ -848,17 +848,15 @@ Future<T?> showMenu<T>({
   assert(captureInheritedThemes != null);
   assert(debugCheckHasMaterialLocalizations(context));
 
-  String? label;
   switch (Theme.of(context)!.platform) {
     case TargetPlatform.iOS:
     case TargetPlatform.macOS:
-      label = semanticLabel;
       break;
     case TargetPlatform.android:
     case TargetPlatform.fuchsia:
     case TargetPlatform.linux:
     case TargetPlatform.windows:
-      label = semanticLabel ?? MaterialLocalizations.of(context)?.popupMenuLabel;
+      semanticLabel ??= MaterialLocalizations.of(context).popupMenuLabel;
   }
 
   return Navigator.of(context, rootNavigator: useRootNavigator)!.push(_PopupMenuRoute<T>(
@@ -866,10 +864,10 @@ Future<T?> showMenu<T>({
     items: items,
     initialValue: initialValue,
     elevation: elevation,
-    semanticLabel: label,
+    semanticLabel: semanticLabel,
     theme: Theme.of(context, shadowThemeOnly: true),
     popupMenuTheme: PopupMenuTheme.of(context),
-    barrierLabel: MaterialLocalizations.of(context)!.modalBarrierDismissLabel,
+    barrierLabel: MaterialLocalizations.of(context).modalBarrierDismissLabel,
     shape: shape,
     color: color,
     showMenuContext: context,
@@ -1142,7 +1140,7 @@ class PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
 
     if (widget.child != null)
       return Tooltip(
-        message: widget.tooltip ?? MaterialLocalizations.of(context)!.showMenuTooltip,
+        message: widget.tooltip ?? MaterialLocalizations.of(context).showMenuTooltip,
         child: InkWell(
           onTap: widget.enabled ? showButtonMenu : null,
           canRequestFocus: _canRequestFocus,
@@ -1153,7 +1151,7 @@ class PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
     return IconButton(
       icon: widget.icon ?? _getIcon(Theme.of(context)!.platform),
       padding: widget.padding,
-      tooltip: widget.tooltip ?? MaterialLocalizations.of(context)!.showMenuTooltip,
+      tooltip: widget.tooltip ?? MaterialLocalizations.of(context).showMenuTooltip,
       onPressed: widget.enabled ? showButtonMenu : null,
     );
   }

packages/flutter/lib/src/material/refresh_indicator.dart

@@ -467,7 +467,7 @@ class RefreshIndicatorState extends State<RefreshIndicator> with TickerProviderS
                   animation: _positionController,
                   builder: (BuildContext context, Widget? child) {
                     return RefreshProgressIndicator(
-                      semanticsLabel: widget.semanticsLabel ?? MaterialLocalizations.of(context)!.refreshIndicatorSemanticLabel,
+                      semanticsLabel: widget.semanticsLabel ?? MaterialLocalizations.of(context).refreshIndicatorSemanticLabel,
                       semanticsValue: widget.semanticsValue,
                       value: showIndeterminateIndicator ? null : _value.value,
                       valueColor: _valueColor,

packages/flutter/lib/src/material/reorderable_list.dart

@@ -402,7 +402,7 @@ class _ReorderableListContentState extends State<_ReorderableListContent> with T
       // before index+2, which is after the space at index+1.
       void moveAfter() => reorder(index, index + 2);
 
-      final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+      final MaterialLocalizations localizations = MaterialLocalizations.of(context);
 
       // If the item can move to before its current position in the list.
       if (index > 0) {

packages/flutter/lib/src/material/search.dart

@@ -479,7 +479,7 @@ class _SearchPageState<T> extends State<_SearchPage<T>> {
     assert(debugCheckHasMaterialLocalizations(context));
     final ThemeData theme = widget.delegate.appBarTheme(context);
     final String searchFieldLabel = widget.delegate.searchFieldLabel
-      ?? MaterialLocalizations.of(context)!.searchFieldLabel;
+      ?? MaterialLocalizations.of(context).searchFieldLabel;
     final TextStyle? searchFieldStyle = widget.delegate.searchFieldStyle
       ?? theme.inputDecorationTheme.hintStyle;
     Widget? body;

packages/flutter/lib/src/material/stepper.dart

@@ -406,7 +406,7 @@ class _StepperState extends State<Stepper> with TickerProviderStateMixin {
 
     final ThemeData themeData = Theme.of(context)!;
     final ColorScheme colorScheme = themeData.colorScheme;
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
 
     const OutlinedBorder buttonShape = RoundedRectangleBorder(borderRadius: BorderRadius.all(Radius.circular(2)));
     const EdgeInsets buttonPadding = EdgeInsets.symmetric(horizontal: 16.0);

packages/flutter/lib/src/material/tabs.dart

@@ -1030,7 +1030,7 @@ class _TabBarState extends State<TabBar> {
       }
       return true;
     }());
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     if (_controller!.length == 0) {
       return Container(
         height: _kTabHeight + widget.indicatorWeight,

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

@@ -865,7 +865,7 @@ class _TextFieldState extends State<TextField> with RestorationMixin implements
   bool get _hasError => widget.decoration?.errorText != null || _hasIntrinsicError;
 
   InputDecoration _getEffectiveDecoration() {
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final ThemeData themeData = Theme.of(context)!;
     final InputDecoration effectiveDecoration = (widget.decoration ?? const InputDecoration())
       .applyDefaults(themeData.inputDecorationTheme)

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

@@ -183,7 +183,7 @@ class _TextSelectionToolbarState extends State<_TextSelectionToolbar> with Ticke
       return const SizedBox(width: 0.0, height: 0.0);
     }
 
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final List<_ItemData> itemDatas = <_ItemData>[
       if (widget.handleCut != null)
         _ItemData(widget.handleCut!, localizations.cutButtonLabel),

packages/flutter/lib/src/material/theme.dart

@@ -132,7 +132,7 @@ class Theme extends StatelessWidget {
       return inheritedTheme.theme.data;
     }
 
-    final MaterialLocalizations? localizations = MaterialLocalizations.of(context);
+    final MaterialLocalizations? localizations = Localizations.of<MaterialLocalizations>(context, MaterialLocalizations);
     final ScriptCategory category = localizations?.scriptCategory ?? ScriptCategory.englishLike;
     final ThemeData theme = inheritedTheme?.theme.data ?? _kFallbackTheme;
     return ThemeData.localize(theme, theme.typography.geometryThemeFor(category));

packages/flutter/lib/src/material/time.dart

@@ -104,7 +104,7 @@ class TimeOfDay {
   String format(BuildContext context) {
     assert(debugCheckHasMediaQuery(context));
     assert(debugCheckHasMaterialLocalizations(context));
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     return localizations.formatTimeOfDay(
       this,
       alwaysUse24HourFormat: MediaQuery.of(context)!.alwaysUse24HourFormat,

packages/flutter/lib/src/material/time_picker.dart

@@ -134,7 +134,7 @@ class _TimePickerHeader extends StatelessWidget {
   Widget build(BuildContext context) {
     assert(debugCheckHasMediaQuery(context));
     final ThemeData themeData = Theme.of(context)!;
-    final TimeOfDayFormat timeOfDayFormat = MaterialLocalizations.of(context)!.timeOfDayFormat(
+    final TimeOfDayFormat timeOfDayFormat = MaterialLocalizations.of(context).timeOfDayFormat(
       alwaysUse24HourFormat: MediaQuery.of(context)!.alwaysUse24HourFormat,
     );
 
@@ -241,7 +241,7 @@ class _TimePickerHeader extends StatelessWidget {
         children: <Widget>[
           const SizedBox(height: 16.0),
           Text(
-            helpText ?? MaterialLocalizations.of(context)!.timePickerDialHelpText,
+            helpText ?? MaterialLocalizations.of(context).timePickerDialHelpText,
             style: TimePickerTheme.of(context).helpTextStyle ?? themeData.textTheme.overline,
           ),
           controls,
@@ -322,7 +322,7 @@ class _HourControl extends StatelessWidget {
   Widget build(BuildContext context) {
     assert(debugCheckHasMediaQuery(context));
     final bool alwaysUse24HourFormat = MediaQuery.of(context)!.alwaysUse24HourFormat;
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final String formattedHour = localizations.formatHour(
       fragmentContext.selectedTime,
       alwaysUse24HourFormat: alwaysUse24HourFormat,
@@ -432,7 +432,7 @@ class _MinuteControl extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final String formattedMinute = localizations.formatMinute(fragmentContext.selectedTime);
     final TimeOfDay nextMinute = fragmentContext.selectedTime.replacing(
       minute: (fragmentContext.selectedTime.minute + 1) % TimeOfDay.minutesPerHour,
@@ -493,7 +493,7 @@ class _DayPeriodControl extends StatelessWidget {
       case TargetPlatform.fuchsia:
       case TargetPlatform.linux:
       case TargetPlatform.windows:
-        _announceToAccessibility(context, MaterialLocalizations.of(context)!.anteMeridiemAbbreviation);
+        _announceToAccessibility(context, MaterialLocalizations.of(context).anteMeridiemAbbreviation);
         break;
       case TargetPlatform.iOS:
       case TargetPlatform.macOS:
@@ -511,7 +511,7 @@ class _DayPeriodControl extends StatelessWidget {
       case TargetPlatform.fuchsia:
       case TargetPlatform.linux:
       case TargetPlatform.windows:
-        _announceToAccessibility(context, MaterialLocalizations.of(context)!.postMeridiemAbbreviation);
+        _announceToAccessibility(context, MaterialLocalizations.of(context).postMeridiemAbbreviation);
         break;
       case TargetPlatform.iOS:
       case TargetPlatform.macOS:
@@ -522,7 +522,7 @@ class _DayPeriodControl extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final MaterialLocalizations materialLocalizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations materialLocalizations = MaterialLocalizations.of(context);
     final ColorScheme colorScheme = Theme.of(context)!.colorScheme;
     final TimePickerThemeData timePickerTheme = TimePickerTheme.of(context);
     final bool isDark = colorScheme.brightness == Brightness.dark;
@@ -941,7 +941,7 @@ class _DialState extends State<_Dial> with SingleTickerProviderStateMixin {
     super.didChangeDependencies();
     assert(debugCheckHasMediaQuery(context));
     themeData = Theme.of(context)!;
-    localizations = MaterialLocalizations.of(context)!;
+    localizations = MaterialLocalizations.of(context);
     media = MediaQuery.of(context)!;
   }
 
@@ -1409,7 +1409,7 @@ class _TimePickerInputState extends State<_TimePickerInput> {
   Widget build(BuildContext context) {
     assert(debugCheckHasMediaQuery(context));
     final MediaQueryData media = MediaQuery.of(context)!;
-    final TimeOfDayFormat timeOfDayFormat = MaterialLocalizations.of(context)!.timeOfDayFormat(alwaysUse24HourFormat: media.alwaysUse24HourFormat);
+    final TimeOfDayFormat timeOfDayFormat = MaterialLocalizations.of(context).timeOfDayFormat(alwaysUse24HourFormat: media.alwaysUse24HourFormat);
     final bool use24HourDials = hourFormat(of: timeOfDayFormat) != HourFormat.h;
     final ThemeData theme = Theme.of(context)!;
     final TextStyle hourMinuteStyle = TimePickerTheme.of(context).hourMinuteTextStyle ?? theme.textTheme.headline2!;
@@ -1420,7 +1420,7 @@ class _TimePickerInputState extends State<_TimePickerInput> {
         crossAxisAlignment: CrossAxisAlignment.start,
         children: <Widget>[
           Text(
-            widget.helpText ?? MaterialLocalizations.of(context)!.timePickerInputHelpText,
+            widget.helpText ?? MaterialLocalizations.of(context).timePickerInputHelpText,
             style: TimePickerTheme.of(context).helpTextStyle ?? theme.textTheme.overline,
           ),
           const SizedBox(height: 16.0),
@@ -1458,7 +1458,7 @@ class _TimePickerInputState extends State<_TimePickerInput> {
                           if (!hourHasError && !minuteHasError)
                             ExcludeSemantics(
                               child: Text(
-                                MaterialLocalizations.of(context)!.timePickerHourLabel,
+                                MaterialLocalizations.of(context).timePickerHourLabel,
                                 style: theme.textTheme.caption,
                                 maxLines: 1,
                                 overflow: TextOverflow.ellipsis,
@@ -1488,7 +1488,7 @@ class _TimePickerInputState extends State<_TimePickerInput> {
                           if (!hourHasError && !minuteHasError)
                             ExcludeSemantics(
                               child: Text(
-                                MaterialLocalizations.of(context)!.timePickerMinuteLabel,
+                                MaterialLocalizations.of(context).timePickerMinuteLabel,
                                 style: theme.textTheme.caption,
                                 maxLines: 1,
                                 overflow: TextOverflow.ellipsis,
@@ -1512,7 +1512,7 @@ class _TimePickerInputState extends State<_TimePickerInput> {
           ),
           if (hourHasError || minuteHasError)
             Text(
-              MaterialLocalizations.of(context)!.invalidTimeLabel,
+              MaterialLocalizations.of(context).invalidTimeLabel,
               style: theme.textTheme.bodyText2!.copyWith(color: theme.colorScheme.error),
             )
           else
@@ -1548,7 +1548,7 @@ class _HourTextField extends StatelessWidget {
       isHour: true,
       autofocus: autofocus,
       style: style,
-      semanticHintText: MaterialLocalizations.of(context)!.timePickerHourLabel,
+      semanticHintText: MaterialLocalizations.of(context).timePickerHourLabel,
       validator: validator,
       onSavedSubmitted: onSavedSubmitted,
       onChanged: onChanged,
@@ -1579,7 +1579,7 @@ class _MinuteTextField extends StatelessWidget {
       isHour: false,
       autofocus: autofocus,
       style: style,
-      semanticHintText: MaterialLocalizations.of(context)!.timePickerMinuteLabel,
+      semanticHintText: MaterialLocalizations.of(context).timePickerMinuteLabel,
       validator: validator,
       onSavedSubmitted: onSavedSubmitted,
     );
@@ -1632,7 +1632,7 @@ class _HourMinuteTextFieldState extends State<_HourMinuteTextField> {
 
   String get _formattedValue {
     final bool alwaysUse24HourFormat = MediaQuery.of(context)!.alwaysUse24HourFormat;
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     return !widget.isHour ? localizations.formatMinute(widget.selectedTime) : localizations.formatHour(
       widget.selectedTime,
       alwaysUse24HourFormat: alwaysUse24HourFormat,
@@ -1770,7 +1770,7 @@ class _TimePickerDialogState extends State<_TimePickerDialog> {
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
-    localizations = MaterialLocalizations.of(context)!;
+    localizations = MaterialLocalizations.of(context);
     _announceInitialTimeOnce();
     _announceModeOnce();
   }
@@ -1855,7 +1855,7 @@ class _TimePickerDialogState extends State<_TimePickerDialog> {
       return;
 
     final MediaQueryData media = MediaQuery.of(context)!;
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     _announceToAccessibility(
       context,
       localizations.formatTimeOfDay(widget.initialTime, alwaysUse24HourFormat: media.alwaysUse24HourFormat),
@@ -1957,8 +1957,8 @@ class _TimePickerDialogState extends State<_TimePickerDialog> {
           onPressed: _handleEntryModeToggle,
           icon: Icon(_entryMode == TimePickerEntryMode.dial ? Icons.keyboard : Icons.access_time),
           tooltip: _entryMode == TimePickerEntryMode.dial
-              ? MaterialLocalizations.of(context)!.inputTimeModeButtonLabel
+              ? MaterialLocalizations.of(context).inputTimeModeButtonLabel
-              : MaterialLocalizations.of(context)!.dialModeButtonLabel,
+              : MaterialLocalizations.of(context).dialModeButtonLabel,
         ),
         Expanded(
           child: Container(

packages/flutter/lib/src/material/user_accounts_drawer_header.dart

@@ -134,7 +134,7 @@ class _AccountDetailsState extends State<_AccountDetails> with SingleTickerProvi
     assert(debugCheckHasMaterialLocalizations(context));
 
     final ThemeData theme = Theme.of(context)!;
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
 
     Widget accountDetails = CustomMultiChildLayout(
       delegate: _AccountDetailsLayout(
@@ -356,7 +356,7 @@ class _UserAccountsDrawerHeaderState extends State<UserAccountsDrawerHeader> {
     assert(debugCheckHasMaterialLocalizations(context));
     return Semantics(
       container: true,
-      label: MaterialLocalizations.of(context)!.signedInLabel,
+      label: MaterialLocalizations.of(context).signedInLabel,
       child: DrawerHeader(
         decoration: widget.decoration ?? BoxDecoration(
           color: Theme.of(context)!.primaryColor,

packages/flutter/test/cupertino/app_test.dart

@@ -50,8 +50,8 @@ void main() {
           builder: (BuildContext context) {
             return Column(
               children: <Widget>[
-                Text(CupertinoLocalizations.of(context)!.selectAllButtonLabel),
+                Text(CupertinoLocalizations.of(context).selectAllButtonLabel),
-                Text(CupertinoLocalizations.of(context)!.datePickerMediumDate(
+                Text(CupertinoLocalizations.of(context).datePickerMediumDate(
                   DateTime(2018, 10, 4),
                 )),
               ],

packages/flutter/test/cupertino/debug_test.dart

@@ -0,0 +1,33 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/src/cupertino/debug.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('debugCheckHasCupertinoLocalizations throws', (WidgetTester tester) async {
+    final GlobalKey noLocalizationsAvailable = GlobalKey();
+    final GlobalKey localizationsAvailable = GlobalKey();
+
+    await tester.pumpWidget(
+      Container(
+        key: noLocalizationsAvailable,
+        child: CupertinoApp(
+          home: Container(
+            key: localizationsAvailable,
+          ),
+        ),
+      ),
+    );
+
+    expect(() => debugCheckHasCupertinoLocalizations(noLocalizationsAvailable.currentContext!), throwsA(isAssertionError.having(
+      (AssertionError e) => e.message,
+      'message',
+      contains('No CupertinoLocalizations found'),
+    )));
+
+    expect(debugCheckHasCupertinoLocalizations(localizationsAvailable.currentContext!), isTrue);
+  });
+}

packages/flutter/test/cupertino/localizations_test.dart

@@ -32,4 +32,28 @@ void main() {
 
     expect(localizations.modalBarrierDismissLabel, isNotNull);
   });
+
+  testWidgets('CupertinoLocalizations.of throws', (WidgetTester tester) async {
+    final GlobalKey noLocalizationsAvailable = GlobalKey();
+    final GlobalKey localizationsAvailable = GlobalKey();
+
+    await tester.pumpWidget(
+      Container(
+        key: noLocalizationsAvailable,
+        child: CupertinoApp(
+          home: Container(
+            key: localizationsAvailable,
+          ),
+        ),
+      ),
+    );
+
+    expect(() => CupertinoLocalizations.of(noLocalizationsAvailable.currentContext!), throwsA(isAssertionError.having(
+      (AssertionError e) => e.message,
+      'message',
+      contains('No CupertinoLocalizations found'),
+    )));
+
+    expect(CupertinoLocalizations.of(localizationsAvailable.currentContext!), isA<CupertinoLocalizations>());
+  });
 }

packages/flutter/test/material/app_test.dart

@@ -500,8 +500,8 @@ void main() {
           builder: (BuildContext context) {
             return Column(
               children: <Widget>[
-                Text(MaterialLocalizations.of(context)!.selectAllButtonLabel),
+                Text(MaterialLocalizations.of(context).selectAllButtonLabel),
-                Text(CupertinoLocalizations.of(context)!.selectAllButtonLabel),
+                Text(CupertinoLocalizations.of(context).selectAllButtonLabel),
               ],
             );
           },

packages/flutter/test/material/localizations_test.dart

@@ -102,4 +102,28 @@ void main() {
     expect(localizations.licensesPackageDetailText(2).contains(r'$licensesCount'), isFalse);
     expect(localizations.licensesPackageDetailText(100).contains(r'$licensesCount'), isFalse);
   });
+
+  testWidgets('MaterialLocalizations.of throws', (WidgetTester tester) async {
+    final GlobalKey noLocalizationsAvailable = GlobalKey();
+    final GlobalKey localizationsAvailable = GlobalKey();
+
+    await tester.pumpWidget(
+      Container(
+        key: noLocalizationsAvailable,
+        child: MaterialApp(
+          home: Container(
+            key: localizationsAvailable,
+          ),
+        ),
+      ),
+    );
+
+    expect(() => MaterialLocalizations.of(noLocalizationsAvailable.currentContext!), throwsA(isAssertionError.having(
+      (AssertionError e) => e.message,
+      'message',
+      contains('No MaterialLocalizations found'),
+    )));
+
+    expect(MaterialLocalizations.of(localizationsAvailable.currentContext!), isA<MaterialLocalizations>());
+  });
 }

packages/flutter/test/material/time_picker_test.dart

@@ -65,7 +65,7 @@ Future<Offset?> startPicker(
 }
 
 Future<void> finishPicker(WidgetTester tester) async {
-  final MaterialLocalizations materialLocalizations = MaterialLocalizations.of(tester.element(find.byType(ElevatedButton)))!;
+  final MaterialLocalizations materialLocalizations = MaterialLocalizations.of(tester.element(find.byType(ElevatedButton)));
   await tester.tap(find.text(materialLocalizations.okButtonLabel));
   await tester.pumpAndSettle(const Duration(seconds: 1));
 }