Use @Native annotation on dart2js DOM native classes

tools/dom/templates/html/impl/impl_EventSource.darttemplate

 // 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 $LIBRARYNAME;
 
-$(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
+$(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
   factory $CLASSNAME(String title, {withCredentials: false}) {
     var parsedOptions = {
       'withCredentials': withCredentials,
     };
     return $CLASSNAME._factory$CLASSNAME(title, parsedOptions);
   }
 $!MEMBERS
 }