on Darwin, we can say:
I'm not totally sure how this works, since exit is provided by Darwin, not by Foundation. But somehow importing Foundation imports Darwin as well.
I think for source compatibility, we should support this, because people <http://stackoverflow.com/questions/34186994/swift-2-2-on-linux-use-of-unresolved-identifier-exit> are asking <http://stackoverflow.com/questions/24102157/seeking-an-exit-equivalent-in-swift> why it doesn't work out of the box.
I considered PRing an exit implementation in corelibs-foundation, but I think that might cause the opposite problem–introduce a conflict between Darwin/Glibc exit and Foundation "exit".
Can someone speak to how this is solved in Darwin Foundation and what we should do about it?