This patch introduces a fake IDL interface just to get the benefits of
cycle collection for the JS-to-C++ link we now need for PromiseNativeHandler
(because the SpiderMonkey Promise somehow needs to point to the
PromiseNativeHandler). Now in practice a bunch of our PromiseNativeHandlers
are not cycle collected. That kinda freaks me out, but spot-checking a few
suggests they do not in fact leak (either because they don't form cycles or
because the Promise they're observing always settles and then releases them).
Either way, that's a problem that exists with or without this patch...
