Adding polyfill support for custom elements.

tests/html/html.status

 # 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.
 
 async_window_test: Skip #TODO(gram): investigating
 event_test: Skip  # Issue 1996
 interactive_test: Skip # Must be run manually.
 
 [ $compiler == dart2js && $runtime != drt ]
-custom_elements_test: Skip
 custom/*: Skip
 
 [ $compiler == dart2js && $browser ]
 # Issue 9325 failures
 custom/attribute_changed_callback_test: Fail
 custom/document_register_type_extensions_test: Fail
 
 [ $compiler == none && ($runtime == drt || $runtime == dartium) && $mode == debug && $system == macos]
 audiobuffersourcenode_test: Pass, Fail, Crash # http://crbug.com/256601
 
@@ skipping 369 matching lines @@
 [ $compiler == dart2js && $csp && ($runtime == drt || $runtime == safari || $runtime == ff || $runtime == chrome || $runtime == chromeOnAndroid) ]
 # Note: these tests are all injecting scripts by design.  This is not allowed under CSP.
 event_customevent_test: Fail       # Test cannot run under CSP restrictions.
 js_interop_1_test: Skip            # Test cannot run under CSP restrictions (times out).
 js_interop_2_test: Fail, OK        # Test cannot run under CSP restrictions.
 js_interop_3_test: Skip            # Test cannot run under CSP restrictions (times out).
 js_test: Skip                      # Test cannot run under CSP restrictions (times out).
 postmessage_structured_test: Skip  # Test cannot run under CSP restrictions (times out).
 safe_dom_test: Skip                # Test cannot run under CSP restrictions (times out).
 shadow_dom_layout_test: Fail, OK   # Test cannot run under CSP restrictions.