Type VirtualizedList's ref

Reviewed By: yungsters

Differential Revision: D7215114

fbshipit-source-id: 867a7b897f27d415269c11572708afeb570b4be0

Author: elicwhite

Libraries/Lists/VirtualizedList.js

@@ -974,7 +974,7 @@ class VirtualizedList extends React.PureComponent<Props, State> {
     velocity: 0,
     visibleLength: 0,
   };
-  _scrollRef = (null: any);
+  _scrollRef: ?React.ElementRef<any> = null;
   _sentEndForContentLength = 0;
   _totalCellLength = 0;
   _totalCellsMeasured = 0;