public Stopwatch start() {
		checkState(!isRunning, "This stopwatch is already running.");
		isRunning = true;
		startTick = ticker.read();
		return this;
	}
Starts the stopwatch.

@return this {@code Stopwatch} instance
@throws IllegalStateException if the stopwatch is already running.
shared/src/main/java/com/oasisfeng/perf/Stopwatch.java