onData_(messages) {
    const callbacks = this.callbackMap_[NodeReadableStream.EventType.DATA];
    if (callbacks) {
      this.doMessages_(messages, callbacks);
    }

    const onceCallbacks =
        this.callbackOnceMap_[NodeReadableStream.EventType.DATA];
    if (onceCallbacks) {
      this.doMessages_(messages, onceCallbacks);
    }
    this.callbackOnceMap_[NodeReadableStream.EventType.DATA] = [];
  }
Handles any new data from XHR.

@param {!Array<!Object>} messages New messages, to be delivered in order
   and atomically.
@private