on(eventType, callback) {
    if (eventType == 'data') {
      const newCallback = (response) => {
        response.data = 'Intercepted ' + response.data;
        callback(response);
      };
      this.stream.on(eventType, newCallback);
    } else {
      this.stream.on(eventType, callback);
    }
    return this;
  }
@override
@param {string} eventType
@param {function(?)} callback
@return {!ClientReadableStream<RESPONSE>}