This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
this.pendingProps = pendingProps;
  this.memoizedProps = null;
  this.updateQueue = null;
  this.memoizedState = null;
  this.dependencies = null;
  this.mode = mode;
  
  this.flags = NoFlags;
  this.subtreeFlags = NoFlags;
  this.deletions = null;
  this.lanes = NoLanes;
  this.childLanes = NoLanes;
  this.alternate = null;
  if (enableProfilerTimer) {
    
    
    
    
    
    
    
    
    
    
    
    
    this.actualDuration = Number.NaN;
    this.actualStartTime = Number.NaN;
    this.selfBaseDuration = Number.NaN;
    this.treeBaseDuration = Number.NaN;