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

Examples/UIExplorer/ListViewExample.js

@@ -21,6 +21,7 @@ var {
   ListView,
   TouchableHighlight,
   StyleSheet,
+  RecyclerViewBackedScrollView,
   Text,
   View,
 } = React;
@@ -55,6 +56,7 @@ var ListViewSimpleExample = React.createClass({
         <ListView
           dataSource={this.state.dataSource}
           renderRow={this._renderRow}
+          renderScrollComponent={props => <RecyclerViewBackedScrollView {...props} />}
         />
       </UIExplorerPage>
     );
@@ -62,9 +64,7 @@ var ListViewSimpleExample = React.createClass({
 
   _renderRow: function(rowData: string, sectionID: number, rowID: number) {
     var rowHash = Math.abs(hashCode(rowData));
-    var imgSource = {
-      uri: THUMB_URLS[rowHash % THUMB_URLS.length],
-    };
+    var imgSource = THUMB_URLS[rowHash % THUMB_URLS.length];
     return (
       <TouchableHighlight onPress={() => this._pressRow(rowID)}>
         <View>
@@ -98,18 +98,18 @@ var ListViewSimpleExample = React.createClass({
 });
 
 var THUMB_URLS = [
-  'Thumbnails/like.png',
-  'Thumbnails/dislike.png',
-  'Thumbnails/call.png',
-  'Thumbnails/fist.png',
-  'Thumbnails/bandaged.png',
-  'Thumbnails/flowers.png',
-  'Thumbnails/heart.png',
-  'Thumbnails/liking.png',
-  'Thumbnails/party.png',
-  'Thumbnails/poke.png',
-  'Thumbnails/superlike.png',
-  'Thumbnails/victory.png',
+  require('./Thumbnails/like.png'),
+  require('./Thumbnails/dislike.png'),
+  require('./Thumbnails/call.png'),
+  require('./Thumbnails/fist.png'),
+  require('./Thumbnails/bandaged.png'),
+  require('./Thumbnails/flowers.png'),
+  require('./Thumbnails/heart.png'),
+  require('./Thumbnails/liking.png'),
+  require('./Thumbnails/party.png'),
+  require('./Thumbnails/poke.png'),
+  require('./Thumbnails/superlike.png'),
+  require('./Thumbnails/victory.png'),
   ];
 var LOREM_IPSUM = 'Lorem ipsum dolor sit amet, ius ad pertinax oportere accommodare, an vix civibus corrumpit referrentur. Te nam case ludus inciderint, te mea facilisi adipiscing. Sea id integre luptatum. In tota sale consequuntur nec. Erat ocurreret mei ei. Eu paulo sapientem vulputate est, vel an accusam intellegam interesset. Nam eu stet pericula reprimique, ea vim illud modus, putant invidunt reprehendunt ne qui.';
 

Examples/UIExplorer/UIExplorerList.android.js

@@ -24,6 +24,7 @@ var UIExplorerListBase = require('./UIExplorerListBase');
 
 var COMPONENTS = [
   require('./ImageExample'),
+  require('./ListViewExample'),
   require('./ProgressBarAndroidExample'),
   require('./ScrollViewSimpleExample'),
   require('./SwitchAndroidExample'),