Summary:
The React Native docs list `replaceAtIndex` as a public method of the **NavigatorIOS** component but it is never exposed.
Closes https://github.com/facebook/react-native/pull/10577
Differential Revision: D4168180
Pulled By: hramos
fbshipit-source-id: 6d91d9de8404bb229930d804f8b01b324ec16f24