This is a Dart script that generates a random alphanumeric string of variable length.

main.dart

import 'dart:core';
import 'dart:math';

String getRandString(len) {
  // Initialization of the string
  var a = '';
  // Generate a list of UTF-16 code units that
  // are limited to just numbers and uppercase letters.
  // Thus, we filter out the code units for non-alphanumeric
  // characters (58:64, inclusive).
  var y = Iterable.generate(42, (int x) => 48 + x++)
      .where((int z) => (z < 58) || (z > 64))
      .toList();
  // Grab a random alphanumeric character by generating a random
  // integer between 1 and the length of the UTF code unit list
  // created above and then using that to get the code unit indexed
  // at that integer.
  for (int x = 1; x <= len; x++) {
    int r = Random().nextInt(y.length);
    int w = y[r];
    a += String.fromCharCode(w);
  }
  return a;
}

void main() {
  int x = 10;
  print('Random value of length $x = ${getRandString(x)}');
}