I've been looking at the viewer example code all day and come to the conclusion that it will never actually work. Specifically the viewer example has a lot of message passing code in viewer-integr-messaging.js but if I grep the source and look at every addEventListener() call in the ./src tree I can only find one instance where it's listening for 'message' and that is in listen() in iframe-helper.js which isn't being called if there are no <amp-iframe> elements.

I confirmed this in chrome debugger by dumping all the listeners on the iframe window containing the AMP document and there is nothing there listening for messages.

What am I missing?