Enforcing that custom tag extensions have correct super classes.

Source/bindings/dart/DartCustomElementConstructorBuilder.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 61 matching lines @@
         return false;
     } else {
         CustomElementException::throwException(CustomElementException::PrototypeDoesNotExtendHTMLElementSVGElementPrototype, type, es);
         return false;
     }
 
     AtomicString localName;
     if (!m_extendsTagName.isNull() && !m_extendsTagName.isEmpty()) {
         localName = m_extendsTagName.lower();
 
+        if (!DartUtilities::isTypeSubclassOfTag(m_customType, m_extendsTagName)) {
+            CustomElementException::throwException(CustomElementException::PrototypeDoesNotExtendHTMLElementSVGElementPrototype, type, es);
+            return false;
+        }
+
         // TODO: enable once we pick up Blink version 31
         // if (!Document::isValidName(localName)) {
         //     CustomElementException::throwException(CustomElementException::ExtendsIsInvalidName, type, es);
         //     return false;
         // }
         // if (CustomElement::isValidName(localName)) {
         //     CustomElementException::throwException(CustomElementException::ExtendsIsCustomElementName, type, es);
         //     return false;
         // }
     } else {
@@ skipping 32 matching lines @@
 {
     return m_callbacks->setBinding(definition, DartCustomElementBinding::create(m_customType));
 }
 
 ScriptValue DartCustomElementConstructorBuilder::bindingsReturnValue() const
 {
     // Dart does not return a constructor.
     return ScriptValue();
 }
 } // namespace WebCore