void start()

Starts the Stopwatch.

The elapsed count is increasing monotonically. If the Stopwatch has been stopped, then calling start again restarts it without resetting the elapsed count.

If the Stopwatch is currently running, then calling start does nothing.

Source 

void start() {
  if (isRunning) return;
  if (_start == null) {
    // This stopwatch has never been started.
    _start = _now();
  } else {
    // Restart this stopwatch. Prepend the elapsed time to the current
    // start time.
    _start = _now() - (_stop - _start);
    _stop = null;
  }
}