docs: function joinDirectory() {
    var dirs = [].slice.call(arguments, 1);
    var dir;
    for (var i = 0, len = dirs.length; i < len; i++) {
      dir = dirs[i];
      if (/^\//.test(dir)) {
        // 发现根目录, 直接返回.
        return dir;
      }
    }
    return join.apply(null, [].slice.call(arguments));
  }