Set return pointer when reusing current tree

We always set the return pointer on freshly cloned, work-in-progress
fibers. However, we were neglecting to set them on trees that are reused
from current.

I fixed this in the same path of the complete phase where we reset the
fiber flags.

This is a code smell because it assumes the commit phase is never
concurrent with the render phase. Our eventual goal is to make fibers a
lock free data structure.

Will address further during refactor to alternate model.

packages/react-reconciler/src/ReactFiberCompleteWork.new.js

@@ -738,6 +738,11 @@ function bubbleProperties(completedWork: Fiber) {
         subtreeFlags |= child.subtreeFlags;
         subtreeFlags |= child.flags;
 
+        // Update the return pointer so the tree is consistent. This is a code
+        // smell because it assumes the commit phase is never concurrent with
+        // the render phase. Will address during refactor to alternate model.
+        child.return = completedWork;
+
         child = child.sibling;
       }
     }
@@ -784,6 +789,11 @@ function bubbleProperties(completedWork: Fiber) {
         subtreeFlags |= child.subtreeFlags & StaticMask;
         subtreeFlags |= child.flags & StaticMask;
 
+        // Update the return pointer so the tree is consistent. This is a code
+        // smell because it assumes the commit phase is never concurrent with
+        // the render phase. Will address during refactor to alternate model.
+        child.return = completedWork;
+
         child = child.sibling;
       }
     }