How to solve issue with surrogate string in flutter and UTF-16

gistfile1.txt

  String get _getSubstringWithSurrogatePairs {
    final substring = _text.substring(0, _maxTextLength);
    final List<int> utf16Bytes = [...substring.codeUnits];
    final int lastIndex = utf16Bytes.length - 1;

    if (substring.length >= _maxTextLength) {
      final int lastByte = utf16Bytes[lastIndex];

      if (lastByte >= 56320 && lastByte <= 57343) {
        // Last byte is a lower surrogate
        return _text.substring(0, _maxTextLength);
      } else if (lastByte >= 55296 && lastByte <= 56319) {
        // Last byte is an upper surrogate
        utf16Bytes.removeLast();
        return String.fromCharCodes(utf16Bytes);
      }
    }

    // Last byte is not part of an extended character with surrogate pairs
    return _text.substring(0, _maxTextLength);
  }