Cleaning up event constructors

tools/dom/templates/html/impl/impl_HashChangeEvent.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 html;
+$ANNOTATIONS
+class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
+  factory $CLASSNAME(String type,
+      {bool canBubble: true, bool cancelable: true, String oldUrl,
+      String newUrl}) {
+    var event = document.$dom_createEvent("HashChangeEvent");
+    event.$dom_initHashChangeEvent(type, canBubble, cancelable, oldUrl, newUrl);
+    return event;
+  }
+$!MEMBERS
+}