randomNID function
Get a random id consists by number.
Implementation
String randomNID({int length = 8, bool withTime = false}) {
assert(length >= 1);
final dt = withTime ? datetime() : '';
final r = Random();
final output = StringBuffer();
for (var i = 0; i < length; ++i) {
output.write(r.nextInt(10));
}
return '$dt${output.toString()}';
}