![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  
i
         o
        const nextParent = parentA.return;
        if (nextParent !== null) {
          a = b = nextParent;
          continue;
        }
         If there's no parent, w
        break;
      }
       I
       a
       p
      if (parentA.child === parentB.child) {
        let child = parentA.child;
        while (child) {
          if (child === a) {
             W
            if (isFiberMountedImpl(parentA) !== MOUNTED) {
              throw Error('Unable to find node on an unmounted component.');
            }
            return fiber;
          }
          if (child === b) {
             W
            if (isFiberMountedImpl(parentA) !== MOUNTED) {
              throw Error('Unable to find node on an unmounted component.');
            }
            return alternate;
          }