Add all dart:html tests from the sdk to test/codegen.

test/codegen/lib/html/mouse_event_test.dart

+// Copyright (c) 2013, 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.
+
+library mouse_event_test;
+import 'package:unittest/unittest.dart';
+import 'package:unittest/html_config.dart';
+import 'dart:html';
+
+main() {
+  useHtmlConfiguration();
+
+  test('relatedTarget', () {
+    var event = new MouseEvent('mouseout');
+    expect(event.relatedTarget, isNull);
+
+    event = new MouseEvent('mouseout', relatedTarget: document.body);
+    expect(event.relatedTarget, document.body);
+  });
+}