Revert "Adding constructor to ContentElement."

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/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
-
-main() {
-  useHtmlConfiguration();
-
-  test('constructor', () {
-      var e = new ContentElement();
-      Expect.isTrue(e is ContentElement);
-    });
-}