Cleaning up event constructors

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

 // Copyright (c) 2012, 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 {
   // In JS, canBubble and cancelable are technically required parameters to
   // init*Event. In practice, though, if they aren't provided they simply
   // default to false (since that's Boolean(undefined)).
   //
   // Contrary to JS, we default canBubble and cancelable to true, since that's
   // what people want most of the time anyway.
-  factory $CLASSNAME(String type, Window view, int detail,
-      [bool canBubble = true, bool cancelable = true]) {
+  factory $CLASSNAME(String type,
+      {Window view, int detail: 0, bool canBubble: true,
+      bool cancelable: true}) {
+    if (view == null) {
+      view = window;
+    }
     final e = document.$dom_createEvent("UIEvent");
     e.$dom_initUIEvent(type, canBubble, cancelable, view, detail);
     return e;
   }
 $!MEMBERS
 }