This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
}
  prepareToReadContext(workInProgress, renderLanes);
  const instance = workInProgress.stateNode;
  let shouldUpdate;
  if (instance === null) {
    if (current !== null) {
      
      
      
      
      current.alternate = null;
      workInProgress.alternate = null;
      
      workInProgress.flags |= Placement;
    }
    
    constructClassInstance(workInProgress, Component, nextProps);
    mountClassInstance(workInProgress, Component, nextProps, renderLanes);
    shouldUpdate = true;
  } else if (current === null) {
    
    shouldUpdate = resumeMountClassInstance(
      workInProgress,
      Component,
      nextProps,
      renderLanes,
    );
  } else {
    shouldUpdate = updateClassInstance(
      current,