anchor.diff:  
   void _asyncRunCallbackLoop() {
     while (_nextCallback != null) {
  +    _lastPriorityCallback = null;
       _AsyncCallbackEntry entry = _nextCallback;
       _nextCallback = entry.next;
       if (_nextCallback == null) _lastCallback = null;
  
positive.code: /** Tail of single linked list of pending callbacks. */
  _AsyncCallbackEntry _lastCallback;
  /**
   * Whether we are currently inside the callback loop.
   *
   * If we are inside the loop, we never need to schedule the loop,
  
negative.code:   try {
      _asyncRunCallbackLoop();
    } finally {
      _lastPriorityCallback = null;
      _isInCallbackLoop = false;
      if (_nextCallback != null) _AsyncRun._scheduleImmediate(_asyncRunCallback);
    }
  