Cleaning up event constructors

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

Index: tools/dom/templates/html/impl/impl_MessageEvent.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_MessageEvent.darttemplate b/tools/dom/templates/html/impl/impl_MessageEvent.darttemplate
new file mode 100644
index 0000000000000000000000000000000000000000..8aae7172e7d4e2d8cb450fe48388d8e3b7562f37
--- /dev/null
+++ b/tools/dom/templates/html/impl/impl_MessageEvent.darttemplate
@@ -0,0 +1,23 @@
+// 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 html;
+
+$(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
+  factory $CLASSNAME(String type,
+      {bool canBubble: false, bool cancelable: false, Object data,
+      String origin, String lastEventId,
+      Window source, List messagePorts}) {
+    if (source == null) {
+      source = window;
+    }
+    var event = document.$dom_createEvent("MessageEvent");
+    event.$dom_initMessageEvent(type, canBubble, cancelable, data, origin,
+        lastEventId, source, messagePorts);
+    return event;
+  }
+$!MEMBERS
+}