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

test/codegen/lib/html/mouse_event_test.dart

Index: test/codegen/lib/html/mouse_event_test.dart
diff --git a/test/codegen/lib/html/mouse_event_test.dart b/test/codegen/lib/html/mouse_event_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..38792c27257a0093da2c6c9f518ccfba2a4e401f
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+++ b/test/codegen/lib/html/mouse_event_test.dart
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+// 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);
+  });
+}