Use @Native annotation on dart2js DOM native classes

tools/dom/templates/html/impl/impl_MutationObserver.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.
 
 part of $LIBRARYNAME;
 
-$(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
+$(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
 $!MEMBERS
   /**
    * Checks to see if the mutation observer API is supported on the current
    * platform.
    */
   static bool get supported {
 $if DARTIUM
     return true;
 $else
     return JS('bool',
@@ skipping 76 matching lines @@
     return JS('MutationObserver',
         'new(window.MutationObserver||window.WebKitMutationObserver||'
         'window.MozMutationObserver)(#)',
         convertDartClosureToJS(_wrapBinaryZone(callback), 2));
   }
 $else
   factory MutationObserver(MutationCallback callback) =>
       new MutationObserver._(_wrapBinaryZone(callback));
 $endif
 }