Summary:
JSC's utf16 -> utf8 conversion crashes on encountering bad utf16. Instead, use our own conversion (conveniently copied from fbjni).
Original fix thanks to rigdern (https://github.com/facebook/react-native/pull/9302)
Reviewed By: mhorowitz
Differential Revision: D3746947
fbshipit-source-id: 29887ca720f6a2b074f01f853bad28a083b273bc
Chris Hopman