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

test/codegen/lib/html/custom_element_name_clash_test.dart

+// Copyright (c) 2015, 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 custom_elements_name_clash;
+
+
+import 'dart:async';
+import 'dart:html';
+import 'package:unittest/html_individual_config.dart';
+import 'package:unittest/unittest.dart';
+import 'utils.dart';
+
+
+class CustomElement extends HtmlElement {
+  factory CustomElement() => new Element.tag('x-custom');
+
+  CustomElement.created() : super.created() {
+  }
+
+  // Try to clash with native 'appendChild' method.
+  var appendChild = 123;
+}
+
+main() {
+  useHtmlIndividualConfiguration();
+
+  setUp(() => customElementsReady);
+
+  group('test', () {
+    test('test', () {
+      document.registerElement('x-custom', CustomElement);
+      CustomElement custom = new CustomElement();
+      document.body.children.add(custom);
+      // Will call appendChild in JS.
+      custom.children.add(new DivElement()..text = 'Hello world!'); 
+    });
+  });
+}