How to generate random string in dart?

Option A with charCodes:

import 'dart:math';

String generateRandomString(int len) {
  var r = Random();
  return String.fromCharCodes(List.generate(len, (index) => r.nextInt(33) + 89));
}

Generates random string using visible characters including special ones.

Option B with a predefined string:

import 'dart:math';

String generateRandomString(int len) {
  var r = Random();
  const _chars = 'AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz1234567890';
  return List.generate(len, (index) => _chars[r.nextInt(_chars.length)]).join();
}

Or if you don't want to use a package you can make a simple implementation like:

import 'dart:math';

void main() {
  print(getRandomString(5));  // 5GKjb
  print(getRandomString(10)); // LZrJOTBNGA
  print(getRandomString(15)); // PqokAO1BQBHyJVK
}

const _chars = 'AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz1234567890';
Random _rnd = Random();

String getRandomString(int length) => String.fromCharCodes(Iterable.generate(
    length, (_) => _chars.codeUnitAt(_rnd.nextInt(_chars.length))));

I should add that you should not use this code to generate passwords or other kind of secrets. If you do that, please at least use Random.secure() to create the random generator.