anchor.diff:     * 
      * The [speed] is the duration of the transition in milliseconds. 
      * If no [speed] is provided, the speed attribute of this [Swiper] is used.
  +   * 
  +   * If [noPageChangeEvent] is set to true, no page change event is fired.
      */
     void moveToIndex(int index, {int speed, bool noPageChangeEvent: false}) {
       if (speed == null) {
  
positive.code:         _onPageChange.add(currentIndex);
        }
      }
    }
     
    /**
  
negative.code:     
      // Install transition end listener.
      container.onTransitionEnd.listen((_) {
        _log.finest('Transition ended (with animation): currentIndex=$currentIndex');
        if (_onTransitionEnd != null) {
          _onTransitionEnd.add(currentIndex);
        }
  