add ui.Window fallback to TestViewConfiguration (#27987)

packages/flutter_test/lib/src/binding.dart

@@ -1319,10 +1319,16 @@ class LiveTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
 /// size onto the actual display using the [BoxFit.contain] algorithm.
 class TestViewConfiguration extends ViewConfiguration {
   /// Creates a [TestViewConfiguration] with the given size. Defaults to 800x600.
-  TestViewConfiguration({
+  ///
+  /// If a [window] instance is not provided it defaults to [ui.window].
+  factory TestViewConfiguration({
     Size size = _kDefaultTestViewportSize,
     ui.Window window,
-  })
+  }) {
+    return TestViewConfiguration._(size, window ?? ui.window);
+  }
+
+  TestViewConfiguration._(Size size, ui.Window window)
     : _paintMatrix = _getMatrix(size, window.devicePixelRatio, window),
       _hitTestMatrix = _getMatrix(size, 1.0, window),
       super(size: size);

packages/flutter_test/test/bindings_test.dart

+// Copyright 2019 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/material.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+
+void main() {
+  group(TestViewConfiguration, () {
+    test('is initialized with top-level window if one is not provided', () {
+      // The code below will throw without the default.
+      TestViewConfiguration(size: const Size(1280.0, 800.0));
+    });
+  });
+}