Aside import/export

PLAT-220

@cpennington @doctoryes plz review. I need to fix the pre-existing error which I marked as FIXME using the 4 stage id lookup we discussed at standup, but as far as xml serialization/deserialization goes, this is done.
Or, if you're going to add the combined method to the interface, it shouldn't be abstract, and should have a default implementation that specific runtimes can override with more efficient methods if they have them.