Add constructors to all the event classes.

client/html/src/MessageEvent.dart

 // Copyright (c) 2011, 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.
+interface MessageEvent extends Event factory MessageEventWrappingImplementation {
 
-interface MessageEvent extends Event {
+  MessageEvent(String type, String data, String origin, String lastEventId,
+      Window source, [bool canBubble, bool cancelable, MessagePort port]);
 
   String get data();
 
   String get lastEventId();
 
   MessagePort get messagePort();
 
   String get origin();
 
   Window get source();
-
-  void initMessageEvent(String typeArg, bool canBubbleArg, bool cancelableArg, String dataArg, String originArg, String lastEventIdArg, Window sourceArg, MessagePort messagePort);
 }