docs: function pathToId(file, base, rel) {
      var id = rel ? path.resolve(rel, '..', file) : file;
      id = path.relative(base, id);
      // replace \ with / in windows' path and remove extname in path
      id = id.replace(BACKLASH_RE, '/').replace(JS_EXT_RE, '');
      return id;
  }