![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  
fiberID: number,
    pendingFiberToMessageCountMap: Map<Fiber, Map<string, number>>,
    fiberIDToMessageCountMap: Map<number, Map<string, number>>,
  ) {
    const fiber = idToArbitraryFiberMap.get(fiberID);
    if (fiber != null) {
      
      pendingFiberToErrorsMap.delete(fiber);
      if (fiberIDToMessageCountMap.has(fiberID)) {
        fiberIDToMessageCountMap.delete(fiberID);
        
        fibersWithChangedErrorOrWarningCounts.add(fiber);
        flushPendingEvents();
        updateMostRecentlyInspectedElementIfNecessary(fiberID);
      } else {
        fibersWithChangedErrorOrWarningCounts.delete(fiber);
      }
    }
  }
  function clearErrorsForFiberID(fiberID: number) {
    clearMessageCountHelper(
      fiberID,
      pendingFiberToErrorsMap,
      fiberIDToErrorsMap,
    );
  }
  function clearWarningsForFiberID(fiberID: number) {