code: function Watcher (specs, options) {
    const self = this
    EventEmitter.call(this)
  
    this._options = {
      dest: options.dest || 'target',
      dir: options.dir || '.'
    }
  
    this._promise = Promise.all(specs)
      .filter((spec) => spec && spec.watch)
      .map((spec) => specToWatcherObj(self, spec, self._options))
      .then((watchers) => {
        self._watchers = watchers
      })
  }
docs: /* Watches some glob specs for changes