Fix mocking on native classes

tests/lib_strong/html/html_mock_test.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'dart:async';
 import 'dart:html';
 
 import 'package:expect/minitest.dart';
 
 class Mock {
   noSuchMethod(Invocation i) => document;
 }
 
 @proxy
-class MockWindow extends Mock implements Window {}
+class MockBodyElement extends Mock implements BodyElement {
+  Node append(Node e) => e;
+}
+
+@proxy
+class MockHtmlDocument extends Mock implements HtmlDocument {
+  BodyElement get body => new MockBodyElement();
+}
+
+@proxy
+class MockWindow extends Mock implements Window {
+  Stream<Event> get onBeforeUnload => new Stream.fromIterable([]);
+
+  String name = "MOCK_NAME";
+}
+
+@proxy
+class MockLocation extends Mock implements Location {
+  String href = "MOCK_HREF";
+}
 
 main() {
   test('is', () {
     var win = new MockWindow();
     expect(win is Window, isTrue);
   });
 
   test('getter', () {
     var win = new MockWindow();
     expect(win.document, equals(document));
   });
+
+  test('override', () {
+    Window win = new MockWindow();
+    expect(win.onBeforeUnload != null, isTrue);
+    expect(win.name, equals("MOCK_NAME"));
+  });
+
+  test('override', () {
+    var loc1 = new MockLocation();
+    Location loc2 = loc1;
+    dynamic loc3 = loc1;
+    expect(loc1.href, equals("MOCK_HREF"));
+    loc1.href = "RESET";
+    expect(loc2.href, equals("RESET"));
+    loc2.href = "RESET2";
+    expect(loc3.href, equals("RESET2"));
+  });
+
+  test('method', () {
+    HtmlDocument doc = new MockHtmlDocument();
+    expect(doc.body.append(null), equals(null));
+  });
 }