on(eventType, callback) {
    let callbacks = this.callbackMap_[eventType];
    if (!callbacks) {
      callbacks = [];
      this.callbackMap_[eventType] = callbacks;
    }

    callbacks.push(callback);
    return this;
  }
@override
@param {string} eventType
@param {function(!Object=)} callback
@return {!NodeReadableStream}