![Screenshot 2019-12-16 10 51 05](https://user-images.githubusercontent.com/793565/70934095-fd91ed80-1ff1-11ea-93b5-746e816585ec.png)
There's seems to be circumstances where unmounted DOM/Fibers are kept alive by React DevTools. They're kept alive in `primaryFibers`:
https://github.com/facebook/react/blob/34527063083195558f98108cde10b5d6ad0d6865/packages/react-devtools-shared/src/backend/renderer.js#L772

It seems like a WeakSet would be appropriate and would remove the leak. Otherwise we'd need to understand why recordUnmount isn't called.

CC @bvaughn  
let ReactPriorityLevels: ReactPriorityLevelsType = {
    ImmediatePriority: 99,
    UserBlockingPriority: 98,
    NormalPriority: 97,
    LowPriority: 96,
    IdlePriority: 95,
    NoPriority: 90,
  };
  if (gt(version, '17.0.2')) {
    ReactPriorityLevels = {
      ImmediatePriority: 1,
      UserBlockingPriority: 2,
      NormalPriority: 3,
      LowPriority: 4,
      IdlePriority: 5,
      NoPriority: 0,
    };
  }
  let ReactTypeOfWork: WorkTagMap = ((null: any): WorkTagMap);
  
  
  
  
  
  
  if (gt(version, '17.0.1')) {
    ReactTypeOfWork = {
      CacheComponent: 24, 
      ClassComponent: 1,
      ContextConsumer: 9,