code: @CanIgnoreReturnValue
    public Stopwatch start() {
      checkState(!isRunning, "This stopwatch is already running.");
      isRunning = true;
      startTick = ticker.read();
      return this;
    }
docs: Starts the stopwatch.
  
  @return this {@code Stopwatch} instance
  @throws IllegalStateException if the stopwatch is already running.