This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
target.child = source.child;
  target.sibling = source.sibling;
  target.index = source.index;
  target.ref = source.ref;
  target.pendingProps = source.pendingProps;
  target.memoizedProps = source.memoizedProps;
  target.updateQueue = source.updateQueue;
  target.memoizedState = source.memoizedState;
  target.dependencies = source.dependencies;
  target.mode = source.mode;
  target.flags = source.flags;
  target.subtreeFlags = source.subtreeFlags;
  target.deletions = source.deletions;
  target.lanes = source.lanes;
  target.childLanes = source.childLanes;
  target.alternate = source.alternate;
  if (enableProfilerTimer) {
    target.actualDuration = source.actualDuration;
    target.actualStartTime = source.actualStartTime;
    target.selfBaseDuration = source.selfBaseDuration;
    target.treeBaseDuration = source.treeBaseDuration;
  }
  target._debugID = source._debugID;
  target._debugSource = source._debugSource;
  target._debugOwner = source._debugOwner;
  target._debugNeedsRemount = source._debugNeedsRemount;
  target._debugHookTypes = source._debugHookTypes;
  return target;
}