Dart, неверное имя конструктора?

import 'package:http/http.dart' as http;

class api_model {
  
  String base_url = 'http://*/Callback/';
  String api_key = '*';

  String getCodes()
  {
    this._request('getCodes');
  }

  String _request(String method, {Map<String, String> params})
  {

    http.post(base_url+'/'+method+'/'+api_key, body: params)
        .then((response) {
      print("Response status: ${response.statusCode}");
      print("Response body: ${response.body}");
    });

  }
}

api_model apiModel = new api_model();
apiModel.getCodes();