Quick fix to the return top level problem

This doesn't deal with the fact that work is usually deferred
so this will return null for first render (except in sync tests).

It also doesn't deal with top levels being fragments etc.
This needs to be unified with refs and findDOMNode better.

However, this does expose that we reactComponentExpect and
ReactTestUtils doesn't work very well with Fiber.

Author: sebmarkbage

src/renderers/dom/fiber/ReactDOMFiber.js

@@ -120,11 +120,13 @@ var ReactDOM = {
 
   render(element : ReactElement<any>, container : DOMContainerElement) {
     warnAboutUnstableUse();
+    let root;
     if (!container._reactRootContainer) {
-      container._reactRootContainer = DOMRenderer.mountContainer(element, container);
+      root = container._reactRootContainer = DOMRenderer.mountContainer(element, container);
     } else {
-      DOMRenderer.updateContainer(element, container._reactRootContainer);
+      DOMRenderer.updateContainer(element, root = container._reactRootContainer);
     }
+    return DOMRenderer.getPublicRootInstance(root);
   },
 
   unmountComponentAtNode(container : DOMContainerElement) {

src/renderers/shared/fiber/ReactFiberReconciler.js

@@ -106,8 +106,13 @@ module.exports = function<T, P, I, TI, C>(config : HostConfig<T, P, I, TI, C>) :
 
     performWithPriority,
 
-    getPublicRootInstance(container : OpaqueNode) : (C | null) {
-      return null;
+    getPublicRootInstance(container : OpaqueNode) : (ReactComponent<any> | I | null) {
+      const root : FiberRoot = (container.stateNode : any);
+      const containerFiber = root.current;
+      if (!containerFiber.child) {
+        return null;
+      }
+      return containerFiber.child.stateNode;
     },
 
   };