Adding support checks for partially supported element types and updating tests.

tests/html/contentelement_test.dart

-// Copyright (c) 2012, 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 contentelement_test;
-import '../../pkg/unittest/lib/unittest.dart';
-import '../../pkg/unittest/lib/html_config.dart';
-import 'dart:html';
-
-main() {
-  useHtmlConfiguration();
-
-  var isContentElement =
-      predicate((x) => x is ContentElement, 'is a ContentElement');
-
-  test('constructor', () {
-      var e = new ContentElement();
-      expect(e, isContentElement);
-    });
-}