Pseudo passw generator. Use your set of symbols as 'source' param.

passw-gen.dart

import 'dart:math';

void main() {
  print(generate('qwertyuiop1234567890'));
}

String generate(String source, [int rLength = 15]){
  final l = source.length;
  final extra = '1234567890qwertyuiopasdfghjklzxcvbnm!#*()-=+<>?plmoknijbuhvygctfxrdzesawq0192837465'.split('').toList()..shuffle();
  final extraS = extra.take(5).join();
  final isUpperCase = Random().nextBool();
  final s = '$source${isUpperCase ? extraS.toUpperCase() : extraS}';
  final shuffled = s.split('').toList()..shuffle();
  
  return shuffled.take(min(rLength, l)).join();
}