echoHandler_(payload) {
    assertTrue('outer frame should be connected', this.channel_.isConnected());
    const peer = this.getInnerPeer_();
    assertTrue('child should be connected', peer.isConnected());
    this.channel_.send('response', payload);
  }
The handler function for incoming echo requests.
@param {string} payload The message payload.
@private
@suppress {strictMissingProperties} suppression added to enable type
checking