Sadly this exposes the fact that libuv doesn't allow multiple handles
for a given poll fd. Nor does it raise clear errors until you try to
close them in an order different from the inverse of the one they were
created in. Doing so simply aborts the process when libuv realises that
the watcher it has registered is not the watcher you're closing.

This further means that multiple watchers wont' get the right set of
events delivered, I think (though most of the tests were passing until I
discovered this crash).