Function updateFragment

packages/react-reconciler/src/ReactChildFiber.js:647–679

(
    returnFiber: Fiber,
    current: Fiber | null,
    fragment: Iterable<React$Node>,
    lanes: Lanes,
    key: null | string,
  )

  }

  function updateFragment(
    returnFiber: Fiber,
    current: Fiber | null,
    fragment: Iterable<React$Node>,
    lanes: Lanes,
    key: null | string,
  ): Fiber {
    if (current === null || current.tag !== Fragment) {
      // Insert
      const created = createFiberFromFragment(
        fragment,
        returnFiber.mode,
        lanes,
        key,
      );
      created.return = returnFiber;
      if (__DEV__) {
        // We treat the parent as the owner for stack purposes.
        created._debugOwner = returnFiber;
        created._debugTask = returnFiber._debugTask;
        created._debugInfo = currentDebugInfo;
      }
      return created;
    } else {
      // Update
      const existing = useFiber(current, fragment);
      existing.return = returnFiber;
      if (__DEV__) {
        existing._debugInfo = currentDebugInfo;
      }
      return existing;
    }
  }

  function createChild(
    returnFiber: Fiber,