unstable_batchedUpdates should return value of callback

src/renderers/dom/fiber/ReactDOMFiber.js

@@ -165,8 +165,8 @@ var ReactDOM = {
     return DOMRenderer.findHostInstance(component);
   },
 
-  unstable_batchedUpdates(fn : Function) : void {
-    DOMRenderer.batchedUpdates(fn);
+  unstable_batchedUpdates<A>(fn : () => A) : A {
+    return DOMRenderer.batchedUpdates(fn);
   },
 
 };

src/renderers/shared/fiber/ReactFiberReconciler.js

@@ -68,7 +68,10 @@ export type Reconciler<C, I, TI> = {
   updateContainer(element : ReactElement<any>, container : OpaqueNode) : void,
   unmountContainer(container : OpaqueNode) : void,
   performWithPriority(priorityLevel : PriorityLevel, fn : Function) : void,
-  batchedUpdates(fn: Function) : void,
+  /* eslint-disable no-undef */
+  // FIXME: ESLint complains about type parameter
+  batchedUpdates<A>(fn : () => A) : A,
+  /* eslint-enable no-undef */
 
   // Used to extract the return value from the initial render. Legacy API.
   getPublicRootInstance(container : OpaqueNode) : (ReactComponent<any, any, any> | TI | I | null),

src/renderers/shared/fiber/ReactFiberScheduler.js

@@ -681,11 +681,11 @@ module.exports = function<T, P, I, TI, C>(config : HostConfig<T, P, I, TI, C>) {
     }
   }
 
-  function batchedUpdates(fn: Function) {
+  function batchedUpdates<A>(fn : () => A) : A {
     const prev = shouldBatchUpdates;
     shouldBatchUpdates = true;
     try {
-      fn();
+      return fn();
     } finally {
       shouldBatchUpdates = prev;
     }