json_serializable.dart

@JsonSerializable(nullable: false)
class Person {
  final String firstName;
  final String lastName;
  Person({this.firstName, this.lastName});
  factory Person.fromJson(Map<String, dynamic> json) => _$PersonFromJson(json);
  Map<String, dynamic> toJson() => _$PersonToJson(this);
}

json_serializable.g.dart

Person _$PersonFromJson(Map<String, dynamic> json) {
  return Person(
    firstName: json['firstName'] as String,
    lastName: json['lastName'] as String);
}

Map<String, dynamic> _$PersonToJson(Person instance) => <String, dynamic>{
  'firstName': instance.firstName,
  'lastName': instance.lastName
};