Occurs in Fiber with multiple instances of the React package (which can't share current owner).
existing.return = returnFiber;
          if (__DEV__) {
            existing._debugSource = element._source;
            existing._debugOwner = element._owner;
          }
          return existing;
        } else {
          deleteRemainingChildren(returnFiber, child);
          break;
        }
      } else {
        deleteChild(returnFiber, child);
      }
      child = child.sibling;
    }
    const created = createFiberFromElement(
      element,
      returnFiber.internalContextTag,
      priority,
    );
    created.ref = coerceRef(currentFirstChild, element);
    created.return = returnFiber;
    return created;
  }
  function reconcileSingleCoroutine(
    returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    coroutine: ReactCoroutine,
    priority: PriorityLevel,
  ): Fiber {