A way to wait for Unicorn.call completion (Promise, callback...)

[matiboy]

When using Unicorn.call and other triggers , it would be nice to be able to wait for the completion of the call.
A promise which resolves with the equivalent of $returnValue perhaps?

I see that messageSender.send uses fetch but doesn't return its promise, and there is the added complexity of the debounced queue.

Or is there an existing way that I've missed? I see something about responseJson.calls but not sure if that would do it.

Any thoughts? Thanks!