使用如下解析函数，然后使用Duration的比较方法：

Duration parseDuration(String s) {
  int hours = 0;
  int minutes = 0;
  int micros;
  List<String> parts = s.split(':');
  if (parts.length > 2) {
    hours = int.parse(parts[parts.length - 3]);
  }
  if (parts.length > 1) {
    minutes = int.parse(parts[parts.length - 2]);
  }
  micros = (double.parse(parts[parts.length - 1]) * 1000000).round();
  return Duration(hours: hours, minutes: minutes, microseconds: micros);
}

包duration提供函数parseTime和tryParseTime来解析Duration().toString()获得的持续时间字符串。
用法很简单：

print(parseTime(Duration(hours: 5, seconds: 10, milliseconds: 567).toString()));

使用DateTime和DateFormat的另一种方法：

Duration durationParse(String time) {
  final ts = DateFormat('y-MM-dd').format(DateTime.now());
  final dt = DateTime.parse('$ts $time');
  return Duration(hours: dt.hour, minutes: dt.minute, seconds: dt.second);
}

List<String> timeParts = time.split(':');

Duration duration = Duration(hours: int.parse(timeParts[0]), minutes: int.parse(timeParts[1]), seconds: int.parse(timeParts[2]));