Fix mocking native classes and add a regression test.

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:html';
+
+import 'package:expect/minitest.dart';
+
+class Mock {
+  noSuchMethod(Invocation i) => document;
+}
+
+@proxy
+class MockWindow extends Mock implements Window {}
+
+main() {
+  test('is', () {
+    var win = new MockWindow();
+    expect(win is Window, isTrue);
+  });
+
+  test('getter', () {
+    var win = new MockWindow();
+    expect(win.document, equals(document));
+  });
+}