![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  
console.warn(`Could not find Fiber with id "${id}"`);
      return null;
    }
    const alternate = fiber.alternate;
    if (!alternate) {
       I
      const state = isFiberMountedImpl(fiber);
      if (state === UNMOUNTED) {
        throw Error('Unable to find node on an unmounted component.');
      }
      if (state === MOUNTING) {
        return null;
      }
      return fiber;
    }
     I
     t
     s
    let a: Fiber = fiber;
    let b: Fiber = alternate;
    while (true) {
      const parentA = a.return;
      if (parentA === null) {
         W
        break;
      }
      const parentB = parentA.alternate;
      if (parentB === null) {
         T
         h