Pass current instead of picking it up from alternate

This colocates the reliance on alternate with the scheduler, so
that we have the option to not use this, or more easily break
apart the initial mount phase into an optimized path.

src/renderers/shared/fiber/ReactChildFiber.js

@@ -236,11 +236,10 @@ function cloneSiblings(current : Fiber, workInProgress : Fiber, returnFiber : Fi
   workInProgress.sibling = null;
 }
 
-exports.cloneChildFibers = function(workInProgress : Fiber) {
+exports.cloneChildFibers = function(current : ?Fiber, workInProgress : Fiber) {
   if (!workInProgress.child) {
     return;
   }
-  const current = workInProgress.alternate;
   if (!current || workInProgress.child !== current.child) {
     // If there is no alternate, then we don't need to clone the children.
     // If the children of the alternate fiber is a different set, then we don't

src/renderers/shared/fiber/ReactFiberBeginWork.js

@@ -166,15 +166,15 @@ module.exports = function<T, P, I, C>(config : HostConfig<T, P, I, C>) {
     if (typeof value === 'object' && value && typeof value.render === 'function') {
       // Proceed under the assumption that this is a class instance
       workInProgress.tag = ClassComponent;
-      if (workInProgress.alternate) {
-        workInProgress.alternate.tag = ClassComponent;
+      if (current) {
+        current.tag = ClassComponent;
       }
       value = value.render();
     } else {
       // Proceed under the assumption that this is a functional component
       workInProgress.tag = FunctionalComponent;
-      if (workInProgress.alternate) {
-        workInProgress.alternate.tag = FunctionalComponent;
+      if (current) {
+        current.tag = FunctionalComponent;
       }
     }
     reconcileChildren(current, workInProgress, value);
@@ -225,7 +225,7 @@ module.exports = function<T, P, I, C>(config : HostConfig<T, P, I, C>) {
     //   return null;
     // }
 
-    cloneChildFibers(workInProgress);
+    cloneChildFibers(current, workInProgress);
     markChildAsProgressed(current, workInProgress, priorityLevel);
     return workInProgress.child;
   }

src/renderers/shared/fiber/ReactFiberCommitWork.js

@@ -28,7 +28,7 @@ module.exports = function<T, P, I, C>(config : HostConfig<T, P, I, C>) {
   const updateContainer = config.updateContainer;
   const commitUpdate = config.commitUpdate;
 
-  function commitWork(finishedWork : Fiber) : void {
+  function commitWork(current : ?Fiber, finishedWork : Fiber) : void {
     switch (finishedWork.tag) {
       case ClassComponent: {
         // TODO: Fire componentDidMount/componentDidUpdate, update refs
@@ -43,14 +43,13 @@ module.exports = function<T, P, I, C>(config : HostConfig<T, P, I, C>) {
         return;
       }
       case HostComponent: {
-        if (finishedWork.stateNode == null || !finishedWork.alternate) {
+        if (finishedWork.stateNode == null || !current) {
           throw new Error('This should only be done during updates.');
         }
         // Commit the work prepared earlier.
         const child = finishedWork.child;
         const children = (child && !child.sibling) ? (child.output : ?Fiber | I) : child;
         const newProps = finishedWork.memoizedProps;
-        const current = finishedWork.alternate;
         const oldProps = current.memoizedProps;
         const instance : I = finishedWork.stateNode;
         commitUpdate(instance, oldProps, newProps, children);

src/renderers/shared/fiber/ReactFiberScheduler.js

@@ -89,7 +89,8 @@ module.exports = function<T, P, I, C>(config : HostConfig<T, P, I, C>) {
     // TODO: Error handling.
     let effectfulFiber = finishedWork.firstEffect;
     while (effectfulFiber) {
-      commitWork(effectfulFiber);
+      const current = effectfulFiber.alternate;
+      commitWork(current, effectfulFiber);
       const next = effectfulFiber.nextEffect;
       // Ensure that we clean these up so that we don't accidentally keep them.
       // I'm not actually sure this matters because we can't reset firstEffect