watcher.unref()
当被调用时，活动的 <fs.FSWatcher> 对象不需要 Node.js 事件循环保持活跃。如果没有其他活动使事件循环继续运行，进程可能会在 <fs.FSWatcher> 对象的回调被调用之前退出。多次调用 watcher.unref() 将不会产生任何效果。
【When called, the active <fs.FSWatcher> object will not require the Node.js
event loop to remain active. If there is no other activity keeping the
event loop running, the process may exit before the <fs.FSWatcher> object's
callback is invoked. Calling watcher.unref() multiple times will have
no effect.】