Change scripts to avoid generating things that bloat the compiled code

tools/dom/templates/html/impl/impl_MessageEvent.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.
 
 // WARNING: Do not edit - generated code.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
   factory $CLASSNAME(String type,
@@ skipping 30 matching lines @@
   // TODO(alanknight): This really should be generated by the
   // _OutputConversion in the systemnative.py script, but that doesn't
   // use those conversions right now, so do this as a one-off.
   @DomName('MessageEvent.data')
   @DocsEditable()
   dynamic get data => convertNativeToDart_SerializedScriptValue(this._get_data);
 
   @JSName('data')
   @DomName('MessageEvent.data')
   @DocsEditable()
-  @Creates('Null')
-  @Returns('Object|Null')
+  @annotation_Creates_SerializedScriptValue
+  @annotation_Returns_SerializedScriptValue
   final dynamic _get_data;
 
 $endif
 
 $!MEMBERS
 }