[web] Freeze window.defaultRouteName (#15565)

lib/web_ui/lib/src/engine/window.dart

@@ -99,8 +99,14 @@ class EngineWindow extends ui.Window {
   /// Simulates clicking the browser's back button.
   Future<void> webOnlyBack() => _browserHistory.back();
 
+  /// Lazily initialized when the `defaultRouteName` getter is invoked.
+  ///
+  /// The reason for the lazy initialization is to give enough time for the app to set [locationStrategy]
+  /// in `lib/src/ui/initialization.dart`.
+  String _defaultRouteName;
+
   @override
-  String get defaultRouteName => _browserHistory.currentPath;
+  String get defaultRouteName => _defaultRouteName ??= _browserHistory.currentPath;
 
   /// Change the strategy to use for handling browser history location.
   /// Setting this member will automatically update [_browserHistory].

lib/web_ui/test/window_test.dart

+// Copyright 2013 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:test/test.dart';
+import 'package:ui/src/engine.dart';
+
+TestLocationStrategy _strategy;
+TestLocationStrategy get strategy => _strategy;
+set strategy(TestLocationStrategy newStrategy) {
+  window.locationStrategy = _strategy = newStrategy;
+}
+
+void main() {
+  test('window.defaultRouteName should not change', () {
+    strategy = TestLocationStrategy.fromEntry(TestHistoryEntry('initial state', null, '/initial'));
+    expect(window.defaultRouteName, '/initial');
+
+    strategy.replaceState(null, null, '/newpath');
+    expect(window.defaultRouteName, '/initial');
+  });
+}