Change data to $ReadOnlyArray

Reviewed By: leebyron

Differential Revision: D4968884

fbshipit-source-id: 9f202d05f7311a192cd939e6d8d72902a54b03a2

Libraries/Lists/FlatList.js

@@ -62,7 +62,7 @@ type RequiredProps<ItemT> = {
    * For simplicity, data is just a plain array. If you want to use something else, like an
    * immutable list, use the underlying `VirtualizedList` directly.
    */
-  data: ?Array<ItemT>,
+  data: ?$ReadOnlyArray<ItemT>,
 };
 type OptionalProps<ItemT> = {
   /**