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;
}
}