Expose in public interface

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

Libraries/Components/ScrollView/RecyclerViewBackedScrollView.ios.js

@@ -5,4 +5,4 @@
  */
 'use strict';
 
-module.exports = require('UnimplementedView');
+module.exports = require('ScrollView');

Libraries/react-native/react-native.js

@@ -32,6 +32,7 @@ var ReactNative = {
   get SnapshotViewIOS() { return require('SnapshotViewIOS'); },
   get Switch() { return require('Switch'); },
   get PullToRefreshViewAndroid() { return require('PullToRefreshViewAndroid'); },
+  get RecyclerViewBackedScrollView() { return require('RecyclerViewBackedScrollView'); },
   get SwitchAndroid() { return require('SwitchAndroid'); },
   get SwitchIOS() { return require('SwitchIOS'); },
   get TabBarIOS() { return require('TabBarIOS'); },
@@ -101,7 +102,7 @@ var ReactNative = {
     get createFragment() { return require('ReactFragment').create; },
     get update() { return require('update'); },
   },
-  
+
   // Note: this must be placed last to prevent eager
   // evaluation of the getter-wrapped submodules above
   ...require('React'),
@@ -109,11 +110,11 @@ var ReactNative = {
 
 if (__DEV__) {
   Object.defineProperty(ReactNative.addons, 'Perf', {
-    enumerable: true, 
+    enumerable: true,
     get: () => require('ReactDefaultPerf'),
   });
   Object.defineProperty(ReactNative.addons, 'TestUtils', {
-    enumerable: true, 
+    enumerable: true,
     get: () => require('ReactTestUtils'),
   });
 }

Libraries/react-native/react-native.js.flow

@@ -44,6 +44,7 @@ var ReactNative = Object.assign(Object.create(require('React')), {
   SnapshotViewIOS: require('SnapshotViewIOS'),
   Switch: require('Switch'),
   PullToRefreshViewAndroid: require('PullToRefreshViewAndroid'),
+  RecyclerViewBackedScrollView: require('RecyclerViewBackedScrollView'),
   SwitchAndroid: require('SwitchAndroid'),
   SwitchIOS: require('SwitchIOS'),
   TabBarIOS: require('TabBarIOS'),