Generate an array of random numbers not present in a list dart

so i have a list of predefined numbers where each number consists of 6 numbers like this {1,2,3,4,5,6 ; 34,52,3,76,12,4 ; 53,1,4,76,23,5 ; .... }

each individual number in the group of numbers range from 1 to 99,

import 'dart:math';

main() {
  var rng = new Random();
  var l = new List.generate(12, (_) => rng.nextInt(100));
}

i want to generate an array of 6 different random numbers using dart in a way that it is not already present in the list of numbers i already have.