'extends Element'

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;
 
 $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
@@ skipping 150 matching lines @@
       $dom_webkitPointerLockElement;
 
   @DomName('Document.webkitVisibilityState')
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
   @Experimental()
   String get visibilityState => $dom_webkitVisibilityState;
 
   @Experimental
 $if DART2JS
-  void register(String tag, Type custom) {
-    throw new UnsupportedError("Custom DOM types not supported");
+  void register(String tag, Type customElementClass) {
+    _registerCustomElement(JS('', 'window'), this, tag, customElementClass);
   }
 $else
   void register(String tag, Type custom) {
     _Utils.register(tag, custom);
   }
 $endif
 
 $if DART2JS
   @Creates('Null')  // Set from Dart code; does not instantiate a native type.
 $endif
   // Note: used to polyfill <template>
   Document _templateContentsOwner;
 }