anchor.diff:     * of parameters.
      */
     async.Future<HttpResponse> get(String url, {
  -    String data,
       Map<String, dynamic> params,
       Map<String, String> headers,
       bool withCredentials: false,
  
positive.code:    * of parameters.
     */
    async.Future<HttpResponse> head(String url, {
      String data,
      Map<String, dynamic> params,
      Map<String, String> headers,
      bool withCredentials: false,
  
negative.code:     interceptors,
      cache,
      timeout
    }) => call(method: 'GET', url: url, data: null, params: params, headers: headers,
               withCredentials: withCredentials, xsrfHeaderName: xsrfHeaderName,
               xsrfCookieName: xsrfCookieName, interceptors: interceptors, cache: cache,
               timeout: timeout);
  