Fixed to allow only HtmlElement in registerElement

tools/dom/templates/html/impl/impl_HTMLDocument.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.
 
 // WARNING: Do not edit - generated code.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
 $!MEMBERS
@@ skipping 340 matching lines @@
       {String extendsTag}) {
 $if DART2JS
     _registerCustomElement(JS('', 'window'), this, tag, customElementClass,
         extendsTag);
 $else
     // TODO(terry): Need to handle the extendsTag.
 
     // Figure out which DOM class is being extended from the user's Dart class.
     var classMirror = reflectClass(customElementClass);
 
+    var isHtmlElement = classMirror.qualifiedName == #dart.dom.html.HtmlElement;
     var locationUri = classMirror.location.sourceUri.toString();
-    if (locationUri == 'dart:html' || locationUri == 'dart:svg') {
+    if (!isHtmlElement && (locationUri == 'dart:html' || locationUri == 'dart:svg')) {
       throw new DomException.jsInterop("HierarchyRequestError: Cannot register an existing dart:html or dart:svg type.");
     }
 
     if (classMirror.isAbstract) {
       throw new DomException.jsInterop("HierarchyRequestError: Cannot register an abstract class.");
     }
 
     var jsClassName = _getJSClassName(classMirror);
     if (jsClassName == null) {
       // Only components derived from HTML* can be extended.
@@ skipping 124 matching lines @@
   /// parameter must be provided.
   @Experimental()
   ElementUpgrader createElementUpgrader(Type type, {String extendsTag}) {
 $if DART2JS
     return new _JSElementUpgrader(this, type, extendsTag);
 $else
     return new _VMElementUpgrader(this, type, extendsTag);
 $endif
   }
 }