docs: function walk_flat(path, iterator, callback) {
    fs.readdir(path, function (err, files) {
      if (err) {
        if ('ENOENT' === err.code) {
          callback(null);
          return;
        }
  
        callback(err);
        return;
      }
  
      async.forEach(files, function (file, next) {
        iterator(path_join(path, file), next);
      }, callback);
    });
  }