code: func dirURL(path string) *url.URL {
  	path = filepath.ToSlash(path)
  	if path[len(path)-1] != '/' {
  		// trailing slash is important
  		path = path + "/"
  	}
  	return &url.URL{Scheme: "file", Path: path}
  }
docs: // NB: `path` is assumed to be in native-OS path separator form