anchor.diff:   * produce "I see 2 other people in Athens." as output in the default locale.
    *
    * To use a locale other than the default, use the `withLocale` function.
  + * You can set the default locale.
  + *       Intl.defaultLocale = "pt_BR";
  + *
  + * To temporarily use a locale other than the default, use the `withLocale`
  + * function.
    *       var todayString = new DateFormat("pt_BR").format(new DateTime.now());
    *       print(withLocale("pt_BR", () => today(todayString));
    *
  
positive.code:     // but must be a static variable in order to be visible to the Intl.message
      // invocation.
      var oldLocale = getCurrentLocale();
      _defaultLocale = locale;
      var result = message_function();
      _defaultLocale = oldLocale;
      return result;
    }
  
  
negative.code:    * locale.
     */
    static String getCurrentLocale() {
      if (defaultLocale == null) defaultLocale = systemLocale;
      return defaultLocale;
    }
  
    toString() => "Intl($locale)";
  