common/fs/deep-watch.js
function addDir (dir, stat) {
  var mtime = stat.mtime.getTime()
  if (!map[dir] && list.length <= maxListSize) {
    map[dir] = mtime
    list.push(dir)
    clearTimeout(sortList.timer)
    sortList.timer = setTimeout(sortList, checkInterval)
    fs.readdir(dir, function (e, files) {
      if (!e) {
        files.forEach(function (file) {
          watch(dir + '/' + file)
        })
      }
    })
  }
}
function addDir (dir, stat) {
  var mtime = stat.mtime.getTime()
  if (!map[dir] && list.length <= maxListSize) {
    map[dir] = mtime
    list.push(dir)
    clearTimeout(sortList.timer)
    sortList.timer = setTimeout(sortList, checkInterval)
    fs.readdir(dir, function (e, files) {
      if (!e) {
        files.forEach(function (file) {
          watch(dir + '/' + file)
        })
      }
    })
  }
}
Add a watchable directory to the map and list of directories we're watching.