Tightening up custom element registration for dart2js

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;
 
 _callConstructor(constructor) {
   return (receiver) {
     return JS('', '#(#)', constructor, receiver);
   };
@@ skipping 57 matching lines @@
   }
 
   // Workaround for 13190- use an article element to ensure that HTMLElement's
   // interceptor is resolved correctly.
   getNativeInterceptor(new Element.tag('article'));
 
   String baseClassName = findDispatchTagForInterceptorClass(interceptorClass);
   if (baseClassName == null) {
     throw new ArgumentError(type);
   }
-  if (baseClassName == 'Element') baseClassName = 'HTMLElement';
+
+  if (extendsTagName == null) {
+    if (baseClassName != 'HTMLElement') {
+      throw new UnsupportedError('Class must provide extendsTag if base '
+          'native class is not HTMLElement');
+    }
+  } else {
+    if (!JS('bool', '(#.createElement(#) instanceof window[#])',
+        document, extendsTagName, baseClassName)) {
+      throw new UnsupportedError('extendsTag does not match base native class');
+    }
+  }
 
   var baseConstructor = JS('=Object', '#[#]', context, baseClassName);
 
   var properties = JS('=Object', '{}');
 
   JS('void', '#.createdCallback = #', properties,
       JS('=Object', '{value: #}',
           _makeCallbackMethod(_callConstructor(constructor))));
   JS('void', '#.enteredViewCallback = #', properties,
       JS('=Object', '{value: #}', _makeCallbackMethod(_callEnteredView)));
@@ skipping 23 matching lines @@
     }
   }
 
   JS('void', '#.register(#, #)', document, tag, options);
 }
 
 //// Called by Element.created to do validation & initialization.
 void _initializeCustomElement(Element e) {
   // TODO(blois): Add validation that this is only in response to an upgrade.
 }