Fix indexdb regression under optimization

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;
 
 $if DART2JS
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS native "Window,DOMWindow" {
 $else
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
@@ skipping 182 matching lines @@
    * Gets an instance of the Indexed DB factory to being using Indexed DB.
    *
    * Use [IdbFactory.supported] to check if Indexed DB is supported on the
    * current platform.
    */
   @SupportedBrowser(SupportedBrowser.CHROME, '23.0')
   @SupportedBrowser(SupportedBrowser.FIREFOX, '15.0')
   @SupportedBrowser(SupportedBrowser.IE, '10.0')
   @Experimental
   IdbFactory get indexedDB =>
-      JS('IdbFactory',
+      JS('IdbFactory|Null',  // If not supported, returns `null`.
          '#.indexedDB || #.webkitIndexedDB || #.mozIndexedDB',
          this, this, this);
 
   @DomName('Window.console')
   Console get console => Console.safeConsole;
 
   /// Checks if _setImmediate is supported.
   static bool get _supportsSetImmediate =>
       JS('bool', '!!(window.setImmediate)');
 
@@ skipping 109 matching lines @@
       return wrapped.returnValue;
     });
 
     return controller.stream;
   }
 
   String getEventType(EventTarget target) {
     return _eventType;
   }
 }