code: function _folderWritten(dest, source, callback, err) {
    if (err) return callback(err);
    async.each(source.children, _write.bind(null, dest), callback);
  }
docs: Ensures each child of source object may write its data.
  @param {String} dest - Path of current directory from the child's
  point of view.
  @param {Object} source - Description of the current structure to
  be created.
  @param {Function} callback - Callback that expects only a possible
  error.
  @param {*} err - Possible error passed by caller.