null-safety.dart

void main() {
  Person('Larry', birthday: DateTime(1973, 03, 26)).describe();
  Person('Sergey').describe();
}

class Person {
  String firstName;
  DateTime? birthday;

  Person(this.firstName, {this.birthday});

  void describe() {
    print(firstName);
    int? birthyear = birthday?.year;
    if (birthyear != null) {
      print('Born ${DateTime.now().year - birthyear} years ago');
    }
  }
}