Add a check for build methods that return context.widget (#25046)

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

@@ -275,6 +275,14 @@ void debugWidgetBuilderValue(Widget widget, Widget built) {
         'To return an empty space that takes as little room as possible, return "new Container(width: 0.0, height: 0.0)".'
       );
     }
+    if (widget == built) {
+      throw FlutterError(
+        'A build function returned context.widget.\n'
+        'The offending widget is: $widget\n'
+        'Build functions must never return their BuildContext parameter\'s widget or a child that contains "context.widget". '
+        'Doing so introduces a loop in the widget tree that can cause the app to crash.'
+      );
+    }
     return true;
   }());
 }

packages/flutter/test/widgets/build_fail_test.dart

+// Copyright 2018 The Chromium 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_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+
+Future<void> main() async {
+  testWidgets('Build method that returns context.widget throws FlutterError', (WidgetTester tester) async {
+    // Regression test for: https://github.com/flutter/flutter/issues/25041
+    await tester.pumpWidget(
+      Builder(builder: (BuildContext context) => context.widget)
+    );
+    expect(tester.takeException(), isFlutterError);
+  });
+}