Use Set if WeakSet is not available

src/createGridComponent.js

@@ -4,6 +4,7 @@ import memoizeOne from 'memoize-one';
 import { createElement, PureComponent } from 'react';
 import { cancelTimeout, requestTimeout } from './timer';
 import { getScrollbarSize } from './domHelpers';
+import WeakSet from './weakset';
 
 import type { TimeoutID } from './timer';
 

src/createListComponent.js

@@ -3,6 +3,7 @@
 import memoizeOne from 'memoize-one';
 import { createElement, PureComponent } from 'react';
 import { cancelTimeout, requestTimeout } from './timer';
+import WeakSet from './weakset';
 
 import type { TimeoutID } from './timer';
 

src/weakset.js

@@ -0,0 +1,4 @@
+// @flow
+
+const WeaksetExport = typeof WeakSet === 'undefined' ? Set : WeakSet;
+export default WeaksetExport;