void reset()

Resets the elapsed count to zero.

This method does not stop or start the Stopwatch.

Source 

void reset() {
  if (_start == null) return;
  // If [_start] is not null, then the stopwatch had already been started. It
  // may running right now.
  _start = _now();
  if (_stop != null) {
    // The watch is not running. So simply set the [_stop] to [_start] thus
    // having an elapsed time of 0.
    _stop = _start;
  }
}