Adding polyfill support for custom elements.

tools/dom/templates/html/impl/impl_Document.darttemplate

 // 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.
 
 part of $LIBRARYNAME;
 
 /**
  * The base class for all documents.
  *
  * Each web page loaded in the browser has its own [Document] object, which is
@@ skipping 17 matching lines @@
    *
    * [selectors] should be a string using CSS selector syntax.
    *     var items = document.queryAll('.itemClassName');
    *
    * For details about CSS selector syntax, see the
    * [CSS selector specification](http://www.w3.org/TR/css3-selectors/).
    */
   ElementList queryAll(String selectors) {
     return new _FrozenElementList._wrap($dom_querySelectorAll(selectors));
   }
+
+  /// Checks if [register] is supported on the current platform.
+  bool get supportsRegister {
+$if DART2JS
+    return JS('bool', '("register" in #)', this);
+$else
+    return true;
+$endif
+  }
 }