code: func (m UsersAPIAuthMethod) LoginURL(c context.Context, dest string) (string, error) {
  	url, err := user.LoginURL(c, dest)
  	if err != nil {
  		return "", err
  	}
  	if !strings.HasPrefix(url, serviceLoginURL) {
  		if !info.IsDevAppServer(c) {
  			logging.Warningf(c, "Unexpected login URL: %q", url)
  		}
  		return url, nil
  	}
  	// Give the user a choice of existing accounts in their session or the option
  	// to add an account, even if they are currently signed in to exactly one
  	// account.
  	return accountChooserURL + url[len(serviceLoginURL):], nil
  }
docs: // LoginURL returns a URL that, when visited, prompts the user to sign in,
  // then redirects the user to the URL specified by dest.