Function useFiber

docs/external/js/react-dom.development.js:13635–13642

(fiber, pendingProps)

    }

    function useFiber(fiber, pendingProps) {
      // We currently set sibling to null and index to 0 here because it is easy
      // to forget to do before returning it. E.g. for the single child case.
      var clone = createWorkInProgress(fiber, pendingProps);
      clone.index = 0;
      clone.sibling = null;
      return clone;
    }

    function placeChild(newFiber, lastPlacedIndex, newIndex) {
      newFiber.index = newIndex;