Summary:
Allows you to do:
```
var { RecyclerViewBackedScrollView } = require('react-native')
```
Rather than:
```
var RecyclerViewBackedScrollView = require('react-native/Libraries/Components/ScrollView/RecyclerViewBackedScrollView')
```
Also...
- Export `ScrollView` by default rather than `UnimplementedView` for `RecyclerViewBackedScrollView` on iOS -- this makes it easier on the user, so you don't have to always do a conditional for: `if IOS then use ScrollView else use RecyclerViewBackedScrollView`. I can't think of a case where this would lead to undesirable behaviour.
- Add `RecyclerViewBackedScrollView` to `MainReactPackage`
- Fix an issue with `MapView` that threw a red-screen when trying to access constants on Android because there is no `MapView` in open source and MapView.js doesn't have a platform extension.
Closes https://github.com/facebook/react-native/pull/4514
Reviewed By: svcscm
Differential Revision: D2753466
Pulled By: mkonicek
fb-gh-sync-id: 0b6e2133975c911d5117e7531cb9093faf314c52
Brent Vatne
Krzysztof Magiera