This is simple; look at how it is done in random_range.dart:

import 'dart:math';

var now = new DateTime.now();
Random rnd = new Random();
Random rnd2 = new Random(now.millisecondsSinceEpoch);

void main() {
  int min = 13, max = 42;
  int r = min + rnd.nextInt(max - min);
  print("$r is in the range of $min and $max"); // e.g. 31
  // used as a function nextInter:
  print("${nextInter(min, max)}"); // for example: 17

  int r2 = min + rnd2.nextInt(max - min);
  print("$r2 is in the range of $min and $max"); // e.g. 33
}

The Random class in dart:math has a method nextInt(int max), which returns a random positive integer between 0 and max (not included). There is no built-in function for our question but it is very easy, as shown in the previous example. If you need this often, use a function nextInter for it, as shown in the following code:

int nextInter(int min, int max) {
  Random rnd = new Random();
  return min + rnd.nextInt(max - min);
}

The variable rnd2 shows another constructor of Random, which takes an integer as a seed for the pseudo-random calculation of nextInt. Using a seed makes for better randomness, and should be used if you need many random values.