'_ $ visited' property on JavaScript object on return of mapv () function from Obervable

I have a WebService implemented with JAX-RS and I'm consuming it with a service ( utilizando Angular 2 )

Follow the service:

@Injectable()
export class ModeloService {

  constructor(private http: Http) {
  }

  getAll(): Observable<Modelo[]> {
    return this.http.get(myEnvironment.MODELO_SERVICE_PATH)
      .map(res => <Modelo[]> res.json());
  }

}

Modelo is this my class:

export class Modelo{
  id: number;
  type: string;
  size: number;

  constructor() {

  }

}

It happens that the return of the function map() , mounts the following object JavaScript , that is, it inserts this property _$visited : true:

{
  "id": 28,
  "type": "type",
  "size": null,
  "_$visited": true
}

This "