![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  
pendingOperations.length = 0;
    }, 1000);
  }
  function reevaluateErrorsAndWarnings() {
    fibersWithChangedErrorOrWarningCounts.clear();
    fiberIDToErrorsMap.forEach((countMap, fiberID) => {
      const fiber = idToArbitraryFiberMap.get(fiberID);
      if (fiber != null) {
        fibersWithChangedErrorOrWarningCounts.add(fiber);
      }
    });
    fiberIDToWarningsMap.forEach((countMap, fiberID) => {
      const fiber = idToArbitraryFiberMap.get(fiberID);
      if (fiber != null) {
        fibersWithChangedErrorOrWarningCounts.add(fiber);
      }
    });
    recordPendingErrorsAndWarnings();
  }
  function mergeMapsAndGetCountHelper(
    fiber: Fiber,
    fiberID: number,
    pendingFiberToMessageCountMap: Map<Fiber, Map<string, number>>,
    fiberIDToMessageCountMap: Map<number, Map<string, number>>,
  ): number {
    let newCount = 0;
    let messageCountMap = fiberIDToMessageCountMap.get(fiberID);
    const pendingMessageCountMap = pendingFiberToMessageCountMap.get(fiber);
    if (pendingMessageCountMap != null) {
      if (messageCountMap == null) {