watch(watchOptions, handler) {
		if (this.running) {
			return handler(new ConcurrentCompilationError());
		}

		this.running = true;
		this.watchMode = true;
		this.watching = new Watching(this, watchOptions, handler);
		return this.watching;
	}
@param {WatchOptions} watchOptions the watcher's options
@param {RunCallback<Stats>} handler signals when the call finishes
@returns {Watching} a compiler watcher