![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.forEach(count => {
          newCount += count;
        });
      }
    }
    pendingFiberToMessageCountMap.delete(fiber);
    return newCount;
  }
  function recordPendingErrorsAndWarnings() {
    clearPendingErrorsAndWarningsAfterDelay();
    fibersWithChangedErrorOrWarningCounts.forEach(fiber => {
      const fiberID = getFiberIDUnsafe(fiber);
      if (fiberID === null) {
         D
      } else {
        const errorCount = mergeMapsAndGetCountHelper(
          fiber,
          fiberID,
          pendingFiberToErrorsMap,
          fiberIDToErrorsMap,
        );
        const warningCount = mergeMapsAndGetCountHelper(
          fiber,
          fiberID,
          pendingFiberToWarningsMap,
          fiberIDToWarningsMap,
        );