![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  
return null;
    }
     W
    if (
      next.hasOwnProperty('baseState') &&
      next.hasOwnProperty('memoizedState') &&
      next.hasOwnProperty('next') &&
      next.hasOwnProperty('queue')
    ) {
      return null;
    }
    const keys = new Set([...Object.keys(prev), ...Object.keys(next)]);
    const changedKeys = [];
    
    for (const key of keys) {
      if (prev[key] !== next[key]) {
        changedKeys.push(key);
      }
    }
    return changedKeys;
  }
   eslint-disable-next-line no-unused-vars
  function didFiberRender(prevFiber: Fiber, nextFiber: Fiber): boolean {