HostRoot no longer pops context provider in complete phase

Also added an invariant warning to guard against context being popped too many times.

Author: Brian Vaughn

src/renderers/shared/fiber/ReactFiberCompleteWork.js

@@ -202,7 +202,6 @@ module.exports = function<T, P, I, TI, C, CX>(
         return null;
       case HostRoot: {
         workInProgress.memoizedProps = workInProgress.pendingProps;
-        popContextProvider();
         const fiberRoot = (workInProgress.stateNode : FiberRoot);
         if (fiberRoot.pendingContext) {
           fiberRoot.context = fiberRoot.pendingContext;

src/renderers/shared/fiber/ReactFiberContext.js

@@ -77,6 +77,7 @@ function isContextProvider(fiber : Fiber) : boolean {
 exports.isContextProvider = isContextProvider;
 
 function popContextProvider() : void {
+  invariant(index > -1, 'Unexpected context pop');
   contextStack[index] = emptyObject;
   didPerformWorkStack[index] = false;
   index--;