"Learn Dart Before you Flutter" Youtube video solution

main.dart

class Order {
  var _id;
  var _reference;
  var date;
  var code;
  List<String> bookings;
  
  Order(this._id, this._reference, {this.date});
  Order.withDiscount(this._id, this._reference, [this.code]) {
    date = DateTime.now();
  }
  
  getInfo() {
    print('''Your order information:
------------------------
  Id: $_id
  Reference: $_reference
  Date: $date
  ${code != null ? 'Code: $code' : ''}
------------------------''');
  }
}

void main() {
  Order order1 = Order(1, 'ref1', date: DateTime.now());
  Order order2 = Order.withDiscount(2, 'ref2')
    ..code = 'DARTROCKS!'
    ..bookings = ['booking1', 'booking2', 'booking3']
    ..getInfo();
  
  order1.getInfo();
}