watcher.ref()
调用时，请求 Node.js 事件循环在 <fs.FSWatcher> 活跃期间 不 退出。多次调用 watcher.ref() 不会产生任何效果。
【When called, requests that the Node.js event loop not exit so long as the
<fs.FSWatcher> is active. Calling watcher.ref() multiple times will have
no effect.】
默认情况下，所有 <fs.FSWatcher> 对象都是“引用”的，因此通常不需要调用 watcher.ref()，除非之前调用过 watcher.unref()。
【By default, all <fs.FSWatcher> objects are "ref'ed", making it normally
unnecessary to call watcher.ref() unless watcher.unref() had been
called previously.】