Allow VirtualizedListContext to support different component type

Summary:
This change is in preparation of adding a separate `VirtualizedList_EXPERIMENTAL` component.

Both the original, and experimental lists use `VirtualizedListContext`, which itself references back to the VirtualizedList class type. VirtualizedList private methods are currently included in the type system, and are called in other VirtualizedList code (see https://github.com/facebook/react-native/commit/b2f871a6fa9c92dd0712055384b9eca6d828e37d). This prevents Flow from seeing the two classes are compatible if "private" methods change.

My first attempt was to parameterize the context, to allow both `VirtualizedList`, and `VirtualizedList_EXPERIMENTAL` to use the same code without sacrificing type safety or adding further duplication. This added more complexity than it is worth, so I am instead loosening the type on VirtualizedListContext to pass around a more generic handle.

Changelog:
[Internal][Changed] - Allow VirtualizedListContext to support different component type

Reviewed By: javache

Differential Revision: D38017086

fbshipit-source-id: 91e8f6ab2591d3ae9b7f9263711b4a39b78f68e0

Libraries/Lists/VirtualizedList.js

@@ -634,10 +634,11 @@ class VirtualizedList extends React.PureComponent<Props, State> {
   _registerAsNestedChild = (childList: {
     cellKey: string,
     key: string,
-    ref: VirtualizedList,
+    ref: React.ElementRef<typeof React.Component>,
     parentDebugInfo: ListDebugInfo,
     ...
   }): ?ChildListState => {
+    const specificRef = ((childList.ref: any): VirtualizedList);
     // Register the mapping between this child key and the cellKey for its cell
     const childListsInCell =
       this._cellKeysToChildListKeys.get(childList.cellKey) || new Set();
@@ -651,19 +652,20 @@ class VirtualizedList extends React.PureComponent<Props, State> {
           'list. You must pass a unique listKey prop to each sibling list.\n\n' +
           describeNestedLists({
             ...childList,
+            ref: specificRef,
             // We're called from the child's componentDidMount, so it's safe to
             // read the child's props here (albeit weird).
-            horizontal: !!childList.ref.props.horizontal,
+            horizontal: !!specificRef.props.horizontal,
           }),
       );
     }
     this._nestedChildLists.set(childList.key, {
-      ref: childList.ref,
+      ref: specificRef,
       state: null,
     });
 
     if (this._hasInteracted) {
-      childList.ref.recordInteraction();
+      specificRef.recordInteraction();
     }
   };
 

Libraries/Lists/VirtualizedListContext.js

@@ -8,7 +8,6 @@
  * @format
  */
 
-import type VirtualizedList from './VirtualizedList.js';
 import * as React from 'react';
 import {useMemo, useContext} from 'react';
 
@@ -50,12 +49,12 @@ type Context = $ReadOnly<{
     zoomScale: number,
   },
   horizontal: ?boolean,
-  getOutermostParentListRef: () => VirtualizedList,
+  getOutermostParentListRef: () => React.ElementRef<typeof React.Component>,
   getNestedChildState: string => ?ChildListState,
   registerAsNestedChild: ({
     cellKey: string,
     key: string,
-    ref: VirtualizedList,
+    ref: React.ElementRef<typeof React.Component>,
     parentDebugInfo: ListDebugInfo,
   }) => ?ChildListState,
   unregisterAsNestedChild: ({