Summary:
This diff adds a new `unstable_enableLogBox` function to opt-into the new LogBox experience. If LogBox is not enabled early enough, we show an error with instructions.
With this, LogBox can be enabled with:
```
require('react-native').unstable_enableLogBox();
```
Changelog: [General] [Adds] unstable_enableLogBox
Reviewed By: zackargyle, rubennorte
Differential Revision: D18808940
fbshipit-source-id: 4b0234ddc4d1646515bf63110d5b02133780512e
Rick Hanlon
dd8e5f468a