![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  
const id = getFiberIDThrows(fiber);
      const prevContexts = idToContextsMap.has(id)
        ? idToContextsMap.get(id)
        : null;
      const nextContexts = getContextsForFiber(fiber);
      if (prevContexts == null || nextContexts == null) {
        return null;
      }
      const [prevLegacyContext, prevModernContext] = prevContexts;
      const [nextLegacyContext, nextModernContext] = nextContexts;
      switch (getElementTypeForFiber(fiber)) {
        case ElementTypeClass:
          if (prevContexts && nextContexts) {
            if (nextLegacyContext !== NO_CONTEXT) {
              return getChangedKeys(prevLegacyContext, nextLegacyContext);
            } else if (nextModernContext !== NO_CONTEXT) {
              return prevModernContext !== nextModernContext;
            }
          }
          break;
        case ElementTypeFunction:
          if (nextModernContext !== NO_CONTEXT) {
            let prevContext = prevModernContext;
            let nextContext = nextModernContext;
            while (prevContext && nextContext) {
              if (!is(prevContext.memoizedValue, nextContext.memoizedValue)) {