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 b2f871a). 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

Author: NickGerleman

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: ({