您可以将解析后的日期转换为原始格式的字符串，然后比较其是否与输入匹配。

You can convert parsed date to string with original format and then compare if it's matching the input.

void main() {
  var inputs = ['20180101', // -> 2018-01-01 00:00:00.000
                '20181231', // -> 2018-12-31 00:00:00.000
                '20180230', // -> 2018-03-02 00:00:00.000
                '20181301', // -> 2019-01-01 00:00:00.000
                '20181364'];// -> 2019-03-05 00:00:00.000

  inputs.forEach((input) {
    print("$input is valid string: ${isValidDate(input)}");
  });
}

bool isValidDate(String input) {
  final date = DateTime.parse(input);
  final originalFormatString = toOriginalFormatString(date);
  return input == originalFormatString;
}

String toOriginalFormatString(DateTime dateTime) {
  final y = dateTime.year.toString().padLeft(4, '0');
  final m = dateTime.month.toString().padLeft(2, '0');
  final d = dateTime.day.toString().padLeft(2, '0');
  return "$y$m$d";
}