up method Null safety
- {Duration timeStamp = Duration.zero}
 
End the gesture by releasing the pointer.
Implementation
Future<void> up({ Duration timeStamp = Duration.zero }) {
  return TestAsyncUtils.guard<void>(() async {
    assert(_pointer._isDown);
    await _dispatcher(_pointer.up(timeStamp: timeStamp));
    assert(!_pointer._isDown);
  });
}