Fix indexdb regression under optimization

sdk/lib/html/dart2js/html_dart2js.dart

 /// The Dart HTML library.
 library dart.dom.html;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:_collection-dev';
 import 'dart:html_common';
 import 'dart:indexed_db';
 import 'dart:isolate';
 import 'dart:json' as json;
@@ skipping 22593 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',
          '#.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 7284 matching lines @@
       _position = nextPosition;
       return true;
     }
     _current = null;
     _position = _array.length;
     return false;
   }
 
   T get current => _current;
 }