npm install
npm run client:generate

{
  "name": "dart-dio-gen",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "client:generate": "rm -rf client/ && openapi-generator generate --generator-name dart-dio --api-name-suffix Api --model-name-suffix ApiModel --input-spec petstore.yaml --output client/"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "@openapitools/openapi-generator-cli": "^1.0.10-4.2.3"
  }
}

        import 'package:built_value/built_value.dart';
import 'package:built_value/serializer.dart';

part 'user.g.dart';

abstract class User implements Built<User, UserBuilder> {

    
        @nullable
    @BuiltValueField(wireName: r'id')
    int get id;
    
        @nullable
    @BuiltValueField(wireName: r'username')
    String get username;
    
        @nullable
    @BuiltValueField(wireName: r'firstName')
    String get firstName;
    
        @nullable
    @BuiltValueField(wireName: r'lastName')
    String get lastName;
    
        @nullable
    @BuiltValueField(wireName: r'email')
    String get email;
    
        @nullable
    @BuiltValueField(wireName: r'password')
    String get password;
    
        @nullable
    @BuiltValueField(wireName: r'phone')
    String get phone;
    /* User Status */
        @nullable
    @BuiltValueField(wireName: r'userStatus')
    int get userStatus;

    // Boilerplate code needed to wire-up generated code
    User._();

    factory User([updates(UserBuilder b)]) = _$User;
    static Serializer<User> get serializer => _$userSerializer;

}

        import 'package:built_value/built_value.dart';
import 'package:built_value/serializer.dart';

part 'user.g.dart';

abstract class UserApiModel implements Built<UserApiModel, UserBuilder> {

    ...

    // Boilerplate code needed to wire-up generated code
    UserApiModel._();

    factory UserApiModel([updates(UserApiModelBuilder b)]) = _$UserApiModel;
    static Serializer<UserApiModel> get serializer => _$userApiModelSerializer;

}