Dart将简单类转换为扩展

这里我粘贴了我的简单类,我可以把英语数字转换成阿拉伯语数字,我想把它转换成扩展名

class Strings{
  static String replaceFarsiNumber(String input, bool reverse) {
    const english = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'];
    const farsi = ['Û°', 'Û±', 'Û²', 'Û³', 'Û´', 'Ûµ', 'Û¶', 'Û·', 'Û¸', 'Û¹'];

    /*en to fa*/
    if (reverse) {
      for (int i = 0; i < english.length; i++) {
        input = input.replaceAll(english[i], farsi[i]);
      }
    } else {
      for (int i = 0; i < english.length; i++) {
        input = input.replaceAll(farsi[i], english[i]);
      }
    }
    return input;
  }
}

我刚进来的时候 extensions 在dart中,我在 flutter 如何将其转换为扩展:

extension ConvertNum on String {
  static const english = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'];
  static const farsi = ['Û°', 'Û±', 'Û²', 'Û³', 'Û´', 'Ûµ', 'Û¶', 'Û·', 'Û¸', 'Û¹'];

  String changeNUm(String input, bool reverse) {
    /*en to fa*/
    if (reverse) {
      for (int i = 0; i < english.length; i++) {
        input = input.replaceAll(english[i], farsi[i]);
      }
    } else {
      for (int i = 0; i < english.length; i++) {
        input = input.replaceAll(farsi[i], english[i]);
      }
    }
    return input;
  }
}