Summary: Before we flow-typed NativeI18nManager, we defaulted the implementation of I18nManager to something safe when it wasn't available. After the flow-type, it became a requirement that NativeI18nManager be present in the app. This is leading to crashes: T45287329. This diff re-enables the defaults for I18nManager.
Reviewed By: fkgozali
Differential Revision: D15617660
fbshipit-source-id: c3a1c737663a1a4ceae484d0ad6cbf2bd86ffe5f