Add Children.toArray to ReactNative

Summary:
To fix #5769. I believe `ReactNative.Children.toArray` should be accessible now.
Closes https://github.com/facebook/react-native/pull/5780

Reviewed By: svcscm

Differential Revision: D2907876

Pulled By: spicyj

fb-gh-sync-id: 649908843f535a89909425bcdcbc58461854779b

Libraries/ReactNative/ReactNative.js

@@ -84,6 +84,7 @@ var ReactNative = {
     map: ReactChildren.map,
     forEach: ReactChildren.forEach,
     count: ReactChildren.count,
+    toArray: ReactChildren.toArray,
     only: onlyChild
   },
   Component: ReactComponent,