Fix the bug when switching to a null portal child

If pendingProps is null, we do a bailout in beginWork.
This prevents unmounting of the existing child when the new child is null.

We fix this by changing portal fiber's pendingProps to be the portal object itself instead of its children.
This way, it is never null, and thus doesn't cause a false positive in the bailout condition.

scripts/fiber/tests-failing.txt

@@ -18,9 +18,6 @@ src/isomorphic/modern/element/__tests__/ReactJSXElementValidator-test.js
 src/renderers/dom/__tests__/ReactDOMProduction-test.js
 * should throw with an error code in production
 
-src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
-* should reconcile portal children
-
 src/renderers/dom/shared/__tests__/ReactDOM-test.js
 * throws in render() if the mount callback is not a function
 * throws in render() if the update callback is not a function

scripts/fiber/tests-passing.txt

@@ -514,6 +514,7 @@ src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
 * should render one portal
 * should render many portals
 * should render nested portals
+* should reconcile portal children
 * should keep track of namespace across portals (simple)
 * should keep track of namespace across portals (medium)
 * should keep track of namespace across portals (complex)

src/renderers/shared/fiber/ReactChildFiber.js

@@ -341,7 +341,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     } else {
       // Update
       const existing = useFiber(current, priority);
-      existing.pendingProps = portal.children;
+      existing.pendingProps = portal;
       existing.return = returnFiber;
       return existing;
     }
@@ -976,7 +976,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
         ) {
           deleteRemainingChildren(returnFiber, child.sibling);
           const existing = useFiber(child, priority);
-          existing.pendingProps = portal.children;
+          existing.pendingProps = portal;
           existing.return = returnFiber;
           return existing;
         } else {

src/renderers/shared/fiber/ReactFiber.js

@@ -346,7 +346,7 @@ exports.createFiberFromYield = function(yieldNode : ReactYield, priorityLevel :
 
 exports.createFiberFromPortal = function(portal : ReactPortal, priorityLevel : PriorityLevel) : Fiber {
   const fiber = createFiber(HostPortal, portal.key);
-  fiber.pendingProps = portal.children;
+  fiber.pendingProps = portal;
   fiber.pendingWorkPriority = priorityLevel;
   fiber.stateNode = {
     containerInfo: portal.containerInfo,

src/renderers/shared/fiber/ReactFiberBeginWork.js

@@ -325,7 +325,7 @@ module.exports = function<T, P, I, TI, C, CX>(
 
   function updatePortalComponent(current, workInProgress) {
     const priorityLevel = workInProgress.pendingWorkPriority;
-    const nextChildren = workInProgress.pendingProps;
+    const nextChildren = workInProgress.pendingProps.children;
     if (!current) {
       // Portals are special because we don't append the children during mount
       // but at commit. Therefore we need to track insertions which the normal