xdart-test/lib/Bicycle.dart

class Bicycle {
  int age;

  Bicycle({this.age});

  factory Bicycle.fromJson(Map<String, dynamic> json) {
    return Bicycle(
      age: json['age'],
    );
  }

  Map toJson() {
    var data = {
      'key': 100,
    };
    data['age'] = age;
    return data;
  }
}