Summary:
further discussion: should there be a `onClose` or `onClosed` to pair with `onShow`? which would make a workaround for #10471 much easier
Closes https://github.com/facebook/react-native/pull/10669
Differential Revision: D4133832
Pulled By: hramos
fbshipit-source-id: 644a5bb6b9da697c81fc96ae4da196ba5b4050cb