Basically just invoke http://heycam.github.io/webidl/#dfn-convert-idl-to-ecmascript-value and then resolve the promise with the result, at least in the ES binding.

Maybe it's clear enough when people resolve promises with IDL values in their algorithms that this is what's supposed to happen, of course.  Except people keep getting confused about what happens with dictionaries and sequences, since resolving with one will create a new JS object.