Change scripts to avoid generating things that bloat the compiled code

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

-// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2016, 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;
 
+// TODO(alanknight): Provide a nicer constructor that uses named parameters
+// rather than an initialization map.
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
-  factory $CLASSNAME(String type,
-      {bool canBubble: false, bool cancelable: false, Object data,
-      String origin, String lastEventId,
-      Window source, List messagePorts}) {
-    if (source == null) {
-      source = window;
-    }
-$if DART2JS
-    if (!Device.isIE) { // TODO: This if check should be removed once IE
-      // implements the constructor.
-      return JS('MessageEvent', 'new MessageEvent(#, {bubbles: #, cancelable: #, data: #, origin: #, lastEventId: #, source: #, ports: #})',
-          type, canBubble, cancelable, data, origin, lastEventId, source,
-          messagePorts);
-    }
-$endif
-    var event = document._createEvent("MessageEvent");
-    event._initMessageEvent(type, canBubble, cancelable, data, origin,
-        lastEventId, source, messagePorts);
-    return event;
-  }
 
 $if DARTIUM
   // 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')
+  @DomName('ServiceWorkerMessageEvent.data')
   @DocsEditable()
   dynamic get data => convertNativeToDart_SerializedScriptValue(
       _blink.BlinkMessageEvent.instance.data_Getter_(unwrap_jso(this)));
-
 $else
   // 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')
+  @DomName('ServiceWorkerMessageEvent.data')
   @DocsEditable()
   dynamic get data => convertNativeToDart_SerializedScriptValue(this._get_data);
 
   @JSName('data')
-  @DomName('MessageEvent.data')
+  @DomName('ServiceWorkerMessageEvent.data')
   @DocsEditable()
-  @Creates('Null')
-  @Returns('Object|Null')
+  @annotation_Creates_SerializedScriptValue
+  @annotation_Returns_SerializedScriptValue
   final dynamic _get_data;
-
 $endif
 
 $!MEMBERS
 }