Fix `getItemLayout` flow type to include `index`

Summary:
The index is used to make sure the layout corresponds to the correct index, which can get
out of sync if cell layout is cached and then cells are re-ordered.

Reviewed By: bvaughn

Differential Revision: D4554721

fbshipit-source-id: 9acb37f390a6e40ad89f813b78f81b399ec11e9a

Examples/UIExplorer/js/MultiColumnExample.js

@@ -114,7 +114,7 @@ class MultiColumnExample extends React.PureComponent {
       </UIExplorerPage>
     );
   }
-  _getItemLayout(data: any, index: number): {length: number, offset: number} {
+  _getItemLayout(data: any, index: number): {length: number, offset: number, index: number} {
     return getItemLayout(data, index);
   }
   _renderItemComponent = ({item}) => {

Libraries/Experimental/FlatList.js

@@ -74,12 +74,13 @@ type OptionalProps = {
    * you know the height of items a priori. getItemLayout is the most efficient, and is easy to use
    * if you have fixed height items, for example:
    *
-   *   getItemLayout={(data, index) => ({length: ITEM_HEIGHT, offset: ITEM_HEIGHT * index})}
+   *   getItemLayout={(data, index) => ({length: ITEM_HEIGHT, offset: ITEM_HEIGHT * index, index})}
    *
    * Remember to include separator length (height or width) in your offset calculation if you
    * specify `SeparatorComponent`.
    */
-  getItemLayout?: (data: ?Array<Item>, index: number) => {length: number, offset: number},
+  getItemLayout?: (data: ?Array<Item>, index: number) =>
+    {length: number, offset: number, index: number},
   /**
    * If true, renders items next to each other horizontally instead of stacked vertically.
    */

Libraries/Experimental/VirtualizedList.js

@@ -87,7 +87,8 @@ type OptionalProps = {
   disableVirtualization: boolean,
   getItem: (items: any, index: number) => ?Item,
   getItemCount: (items: any) => number,
-  getItemLayout?: (items: any, index: number) => {length: number, offset: number}, // e.g. height, y
+  getItemLayout?: (items: any, index: number) =>
+    {length: number, offset: number, index: number}, // e.g. height, y
   horizontal: boolean,
   initialNumToRender: number,
   keyExtractor: (item: Item, index: number) => string,