PointerUpEvent up({Duration timeStamp: Duration.ZERO })

Create a PointerUpEvent.

By default, the time stamp on the event is Duration.ZERO. You can give a specific time stamp by passing the timeStamp argument.

The object is no longer usable after this method has been called.

Source 

PointerUpEvent up({ Duration timeStamp: Duration.ZERO }) {
  assert(isDown);
  _isDown = false;
  return new PointerUpEvent(
    timeStamp: timeStamp,
    pointer: pointer,
    position: location
  );
}