factorial function Null safety

Implementation

double factorial(double x) {
  if (x >= 70) {
    return double.infinity;
  } else if (x >= 0 && x == x.floorToDouble()) {
    final lim = x.floor();
    double result = 1;
    for (int i = 1; i <= lim; i++) {
      result *= i;
    }
    return result;
  } else {
    return laGamma(x + 1);
  }
}