Adding constructor to ContentElement.

tests/html/html.status

 # 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.
 
 window_open_test: Fail, Pass # http://dartbug.com/5151
 
 event_test: Skip  # Issue 1996
 webgl_1_test: Skip  # Issue 1495
 xmldocument_test: Skip  # XML in dart:html is incomplete
 xmlelement_test: Skip  # XML in dart:html is incomplete
@@ skipping 35 matching lines @@
 # TODO(vsm): Triage DOM tests.
 htmloptionscollection_test: Fail # Issue 3813.
 shadow_dom_test: Skip # No ShadowDOM support except on tip dartium.
 shadow_dom_layout_test: Skip
 unknownelement_test: Fail # Issue 4189
 
 [ $runtime == dartium || $runtime == chrome || $runtime == ie || $runtime == safari || $runtime == ff || $runtime == opera ]
 history_test: Fail
 
 [ $runtime == ie ]
+contentelement_test: Fail   # ie does not support content element.
 form_data_test: Fail # Issue 4793.
 form_element_test: Fail # Issue 4793.
 typed_arrays_1_test: Skip   # ie9 does not have typed arrays.
 typed_arrays_2_test: Skip
 typed_arrays_3_test: Skip
 typed_arrays_4_test: Skip
 typed_arrays_5_test: Skip
 typed_arrays_arraybuffer_test: Skip
 typed_arrays_dataview_test: Skip
 typed_arrays_range_checks_test: Fail
@@ skipping 46 matching lines @@
 node_test: Fail
 svgelement_test: Fail
 svgelement2_test: Fail
 
 [ $runtime == ff ]
 audiocontext_test: Fail      # FF only has Audio element
 css_test: Fail               # No analog to WebKitCSSMatrix
 callbacks_test: Fail         # (webkit|moz|ms)RequestAnimationFrame
 canvas_test: Fail            # No setFillColor() method.  Use fillstyle attribute instead.
 canvas_using_html_test: Fail # No setFillColor() method.  Use fillstyle attribute instead.
+contentelement_test: Fail    # FF does not support content element.
 dart_object_local_storage_test: Skip  # sessionStorage NS_ERROR_DOM_NOT_SUPPORTED_ERR
 documentfragment_test: Fail
 element_test: Fail
 exceptions_test: Fail      # Uses webkitotifications, no analogue in moz
 fileapi_test: Fail         # FF does not support filesystem APIs.
 indexeddb_1_test: Fail     # Need window.mozIndexedDB instead of window.webkitIndexedDB
 indexeddb_2_test: Fail     # Need window.mozIndexedDB instead of window.webkitIndexedDB
 indexeddb_3_test: Fail     # Need window.mozIndexedDB instead of window.webkitIndexedDB
 indexeddb_4_test: Fail     # Need window.mozIndexedDB instead of window.webkitIndexedDB
 # setup code fails. prepare. (DOM callback has errors) Caught [object Event]
@@ skipping 65 matching lines @@
 [ $compiler == dart2js && $runtime == chrome && $system == windows]
 css_test: Pass, Fail # Issue #2823
 
 [ $compiler == dart2js && $runtime == ff && $system == windows]
 js_interop_2_test: Fail # Issue 4658.
 
 [ $compiler == dart2js && $checked && $browser]
 fileapi_test: Fail # TypeError: Object #<DirectoryEntry> has no method 'get$name'
 cross_frame_test: Fail # Caught NoSuchMethodError : method not found: 'is$_WindowImpl'
 window_open_test: Fail # Caught NoSuchMethodError : method not found: 'is$_WindowImpl'