Make WidgetsLocalizations.of non-nullable (#69055)

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

@@ -259,7 +259,7 @@ abstract class CupertinoLocalizations {
   /// CupertinoLocalizations.of(context).anteMeridiemAbbreviation;
   /// ```
   static CupertinoLocalizations of(BuildContext context) {
-    debugCheckHasCupertinoLocalizations(context);
+    assert(debugCheckHasCupertinoLocalizations(context));
     return Localizations.of<CupertinoLocalizations>(context, CupertinoLocalizations)!;
   }
 }

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

@@ -506,7 +506,7 @@ abstract class MaterialLocalizations {
   /// tooltip: MaterialLocalizations.of(context).backButtonTooltip,
   /// ```
   static MaterialLocalizations of(BuildContext context) {
-    debugCheckHasMaterialLocalizations(context);
+    assert(debugCheckHasMaterialLocalizations(context));
     return Localizations.of<MaterialLocalizations>(context, MaterialLocalizations)!;
   }
 }

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

@@ -9,6 +9,7 @@ import 'package:flutter/foundation.dart';
 
 import 'basic.dart';
 import 'framework.dart';
+import 'localizations.dart';
 import 'media_query.dart';
 import 'table.dart';
 
@@ -321,6 +322,44 @@ void debugWidgetBuilderValue(Widget widget, Widget? built) {
   }());
 }
 
+/// Asserts that the given context has a [Localizations] ancestor that contains
+/// a [WidgetsLocalizations] delegate.
+///
+/// To call this function, use the following pattern, typically in the
+/// relevant Widget's build method:
+///
+/// ```dart
+/// assert(debugCheckHasWidgetsLocalizations(context));
+/// ```
+///
+/// Does nothing if asserts are disabled. Always returns true.
+bool debugCheckHasWidgetsLocalizations(BuildContext context) {
+  assert(() {
+    if (Localizations.of<WidgetsLocalizations>(context, WidgetsLocalizations) == null) {
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('No WidgetsLocalizations found.'),
+        ErrorDescription(
+          '${context.widget.runtimeType} widgets require WidgetsLocalizations '
+          'to be provided by a Localizations widget ancestor.'
+        ),
+        ErrorDescription(
+          'The widgets library uses Localizations to generate messages, '
+          'labels, and abbreviations.'
+        ),
+        ErrorHint(
+          'To introduce a WidgetsLocalizations, either use a '
+          'WidgetsApp at the root of your application to include them '
+          'automatically, or add a Localization widget with a '
+          'WidgetsLocalizations delegate.'
+        ),
+        ...context.describeMissingAncestor(expectedAncestorType: WidgetsLocalizations)
+      ]);
+    }
+    return true;
+  }());
+  return true;
+}
+
 /// Returns true if none of the widget library debug variables have been changed.
 ///
 /// This function is used by the test framework to ensure that debug variables

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

@@ -10,6 +10,7 @@ import 'package:flutter/rendering.dart';
 import 'basic.dart';
 import 'binding.dart';
 import 'container.dart';
+import 'debug.dart';
 import 'framework.dart';
 
 // Examples can assume:
@@ -155,7 +156,7 @@ abstract class WidgetsLocalizations {
   /// that encloses the given context.
   ///
   /// This method is just a convenient shorthand for:
-  /// `Localizations.of<WidgetsLocalizations>(context, WidgetsLocalizations)`.
+  /// `Localizations.of<WidgetsLocalizations>(context, WidgetsLocalizations)!`.
   ///
   /// References to the localized resources defined by this class are typically
   /// written in terms of this method. For example:
@@ -163,8 +164,9 @@ abstract class WidgetsLocalizations {
   /// ```dart
   /// textDirection: WidgetsLocalizations.of(context).textDirection,
   /// ```
-  static WidgetsLocalizations? of(BuildContext context) {
-    return Localizations.of<WidgetsLocalizations>(context, WidgetsLocalizations);
+  static WidgetsLocalizations of(BuildContext context) {
+    assert(debugCheckHasWidgetsLocalizations(context));
+    return Localizations.of<WidgetsLocalizations>(context, WidgetsLocalizations)!;
   }
 }
 

packages/flutter/test/widgets/debug_test.dart

@@ -215,6 +215,33 @@ void main() {
     }
   });
 
+  testWidgets('debugCheckHasWidgetsLocalizations throws', (WidgetTester tester) async {
+    final GlobalKey noLocalizationsAvailable = GlobalKey();
+    final GlobalKey localizationsAvailable = GlobalKey();
+
+    await tester.pumpWidget(
+      Container(
+        key: noLocalizationsAvailable,
+        child: WidgetsApp(
+          builder: (BuildContext context, Widget? child) {
+            return Container(
+              key: localizationsAvailable,
+            );
+          },
+          color: const Color(0xFF4CAF50),
+        ),
+      ),
+    );
+
+    expect(() => debugCheckHasWidgetsLocalizations(noLocalizationsAvailable.currentContext!), throwsA(isAssertionError.having(
+      (AssertionError e) => e.message,
+      'message',
+      contains('No WidgetsLocalizations found'),
+    )));
+
+    expect(debugCheckHasWidgetsLocalizations(localizationsAvailable.currentContext!), isTrue);
+  });
+
   test('debugAssertAllWidgetVarsUnset', () {
     debugHighlightDeprecatedWidgets = true;
     late FlutterError error;