nextString method
Implementation
String nextString(int length,
{String charset =
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"}) {
return String.fromCharCodes(Iterable.generate(
length, (_) => charset.codeUnitAt(_random.nextInt(charset.length))));
}