common/fs/deep-watch.js
function startWatches () {
  list.forEach(function (dir, i) {
    if (i < maxFsWatches) {
      try {
        fs.watch(dir, function (op, file) {
          notify(dir + '/' + file)
        })
      } catch (e) {
        // fs.watch is known to be unstable.
      }
    }
  })
}
function startWatches () {
  list.forEach(function (dir, i) {
    if (i < maxFsWatches) {
      try {
        fs.watch(dir, function (op, file) {
          notify(dir + '/' + file)
        })
      } catch (e) {
        // fs.watch is known to be unstable.
      }
    }
  })
}
Iterate over the age-prioritized list, and start fs watches.