Update Dispatcher.js

I think Array.filter expects a boolean return value, am I correct?

examples/todomvc-flux/js/dispatcher/Dispatcher.js

@@ -53,9 +53,9 @@ var _clearPromises = function() {
  * Used below in waitFor().
  * @param {number} index The index within the _promises array
  */
-var _getPromise = function(index) {
-  return _promises[index];
-};
+var _checkPromise = function(_,j){
+  return promiseIndexes.indexOf(j) !== -1;
+}
 
 var Dispatcher = function() {};
 Dispatcher.prototype = merge(Dispatcher.prototype, {
@@ -116,7 +116,7 @@ Dispatcher.prototype = merge(Dispatcher.prototype, {
    * A more robust Dispatcher would issue a warning in this scenario.
    */
   waitFor: function(/*array*/ promiseIndexes, /*function*/ callback) {
-    var selectedPromises = promiseIndexes.filter(_getPromise);
+    var selectedPromises = _promises.filter(_checkPromise)
     Promise.all(selectedPromises).then(callback);
   }