Allow test beds to override defaultTestTimeout (#40806)

packages/flutter_test/lib/src/binding.dart

@@ -839,8 +839,11 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
   @override
   bool get checkIntrinsicSizes => true;
 
+  /// The value of [defaultTestTimeout] can be set to `None` to enable debugging flutter tests where
+  /// we would not want to timeout the test. This is expected to be used by test tooling which
+  /// can detect debug mode.
   @override
-  test_package.Timeout get defaultTestTimeout => const test_package.Timeout(Duration(minutes: 10));
+  test_package.Timeout defaultTestTimeout = const test_package.Timeout(Duration(minutes: 10));
 
   @override
   bool get inTest => _currentFakeAsync != null;

packages/flutter_test/test/bindings_test.dart

@@ -5,7 +5,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
-
+import 'package:test_api/test_api.dart' as test_package;
 
 void main() {
   group(TestViewConfiguration, () {
@@ -14,4 +14,12 @@ void main() {
       TestViewConfiguration(size: const Size(1280.0, 800.0));
     });
   });
+
+  group(AutomatedTestWidgetsFlutterBinding, () {
+    test('allows setting defaultTestTimeout to 5 minutes', () {
+      final AutomatedTestWidgetsFlutterBinding binding = AutomatedTestWidgetsFlutterBinding();
+      binding.defaultTestTimeout = const test_package.Timeout(Duration(minutes: 5));
+      expect(binding.defaultTestTimeout.duration, const Duration(minutes: 5));
+    });
+  });
 }