![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  
b = parentA;
            a = parentB;
            break;
          }
          child = child.sibling;
        }
        if (!didFindChild) {
           S
          child = parentB.child;
          while (child) {
            if (child === a) {
              didFindChild = true;
              a = parentB;
              b = parentA;
              break;
            }
            if (child === b) {
              didFindChild = true;
              b = parentB;
              a = parentA;
              break;
            }
            child = child.sibling;
          }
          if (!didFindChild) {
            throw Error(
              'Child was not found in either parent set. This indicates a bug ' +
                'in React related to the return pointer. Please file an issue.',
            );
          }