Adding polyfill support for custom elements.

tools/dom/src/dart2js_CustomElementSupport.dart

 // 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.
 
 part of dart.dom.html;
 
 _callOnCreated(receiver) {
   return receiver.onCreated();
 }
 
@@ skipping 27 matching lines @@
     throw new ArgumentError(type);
   }
 
   String baseClassName = findDispatchTagForInterceptorClass(interceptorClass);
   if (baseClassName == null) {
     throw new ArgumentError(type);
   }
   if (baseClassName == 'Element') baseClassName = 'HTMLElement';
 
   var baseConstructor = JS('=Object', '#[#]', context, baseClassName);
-  if (JS('bool', "typeof(#) != 'function'", baseConstructor)) {
-    throw new ArgumentError(type);
-  }
 
   var properties = JS('=Object', '{}');
 
   var jsCreatedCallback = _makeCreatedCallbackMethod();
 
   JS('void', '#.createdCallback = #', properties,
       JS('=Object', '{value: #}', jsCreatedCallback));
 
   var baseProto = JS('=Object', '#.prototype', baseConstructor);
   var proto = JS('=Object', 'Object.create(#, #)', baseProto, properties);
 
   var interceptor = JS('=Object', '#.prototype', interceptorClass);
 
   setNativeSubclassDispatchRecord(proto, interceptor);
 
   JS('void', '#.register(#, #)',
       document, tag, JS('', '{prototype: #}', proto));
 }