Allow Window.open to return null in dart2js

tools/dom/templates/html/impl/impl_Window.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.
 
 part of $LIBRARYNAME;
 
 @DocsEditable()
 $if DART2JS
 $(ANNOTATIONS)@Native("Window,DOMWindow")
 $(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
@@ skipping 29 matching lines @@
    * The newest document in this window.
    *
    * ## Other resources
    *
    * * [Loading web
    *   pages](https://html.spec.whatwg.org/multipage/browsers.html)
    *   from WHATWG.
    */
   Document get document => JS('Document', '#.document', this);
 
-  WindowBase _open2(url, name) => JS('Window', '#.open(#,#)', this, url, name);
+  WindowBase _open2(url, name) =>
+      JS('Window|Null', '#.open(#,#)', this, url, name);
 
   WindowBase _open3(url, name, options) =>
-      JS('Window', '#.open(#,#,#)', this, url, name, options);
+      JS('Window|Null', '#.open(#,#,#)', this, url, name, options);
 
   /**
    * Opens a new window.
    *
    * ## Other resources
    *
    * * [Window.open](https://developer.mozilla.org/en-US/docs/Web/API/Window.open)
    *   from MDN.
    * * [Window open](http://docs.webplatform.org/wiki/dom/methods/open)
    *   from WebPlatform.org.
@@ skipping 296 matching lines @@
 
   ElementStream<BeforeUnloadEvent> forElement(Element e, {bool useCapture: false}) {
     return new _ElementEventStreamImpl(e, _eventType, useCapture);
   }
 
   ElementStream<BeforeUnloadEvent> _forElementList(ElementList e,
       {bool useCapture: false}) {
     return new _ElementListEventStreamImpl(e, _eventType, useCapture);
   }
 }