code: public Stopwatch stop() {
      long tick = System.nanoTime();
      isRunning = false;
      elapsedNanos += tick - startTick;
      return this;
    }
docs: Stops the stopwatch. Future reads will return the fixed duration that had
  elapsed up to this point.
  
  @return this {@code Stopwatch} instance
  @throws IllegalStateException if the stopwatch is already stopped.