![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  
messageCountMap = pendingMessageCountMap;
        fiberIDToMessageCountMap.set(fiberID, pendingMessageCountMap);
      } else {
         T
        const refinedMessageCountMap = ((messageCountMap: any): Map<
          string,
          number,
        >);
        pendingMessageCountMap.forEach((pendingCount, message) => {
          const previousCount = refinedMessageCountMap.get(message) || 0;
          refinedMessageCountMap.set(message, previousCount + pendingCount);
        });
      }
    }
    if (!shouldFilterFiber(fiber)) {
      if (messageCountMap != null) {