Constrain data type in getItemLayout callback (#36237)

Summary:
Pull Request resolved: #36237

This changes the data parameter type for `getItemLayout` from a mutable array (too lenient, even before), to `ArrayLike`, which is now the most constrained subset of data which may be passed to a FlatList.

We could do something more exact by adding another generic parameter to FlatList, but that would be likely be noticeably more breaking, since during testing I couldn't manage a pattern that both kept the same minimum number of generic arguments while keeping inference working.

Changelog:
[General][Breaking] - Constrain data type in `getItemLayout` callback

Reviewed By: javache

Differential Revision: D43466967

fbshipit-source-id: 7a1ce717e7d5cc96a58b8d3ad9def6cf6250871f

Libraries/Lists/FlatList.d.ts

@@ -66,7 +66,7 @@ export interface FlatListProps<ItemT> extends VirtualizedListProps<ItemT> {
    */
   getItemLayout?:
     | ((
-        data: Array<ItemT> | null | undefined,
+        data: ArrayLike<ItemT> | null | undefined,
         index: number,
       ) => {length: number; offset: number; index: number})
     | undefined;

Libraries/Lists/FlatList.js

@@ -91,7 +91,7 @@ type OptionalProps<ItemT> = {|
    * specify `ItemSeparatorComponent`.
    */
   getItemLayout?: (
-    data: ?Array<ItemT>,
+    data: ?$ArrayLike<ItemT>,
     index: number,
   ) => {
     length: number,

types/__typetests__/index.tsx

@@ -705,6 +705,21 @@ export class FlatListTest extends React.Component<FlatListProps<number>, {}> {
   }
 }
 
+<FlatList
+  data={[1, 2, 3]}
+  renderItem={null}
+  getItemLayout={(
+    data: ArrayLike<number> | null | undefined,
+    index: number,
+  ) => {
+    return {
+      length: data![index] % 2 === 0 ? 10 : 5,
+      offset: 1234,
+      index,
+    };
+  }}
+/>;
+
 export class SectionListTest extends React.Component<
   SectionListProps<string>,
   {}