![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 fibersWithChangedErrorOrWarningCounts: Set<Fiber> = new Set();
  const pendingFiberToErrorsMap: Map<Fiber, Map<string, number>> = new Map();
  const pendingFiberToWarningsMap: Map<Fiber, Map<string, number>> = new Map();
  
  const fiberIDToErrorsMap: Map<number, Map<string, number>> = new Map();
  const fiberIDToWarningsMap: Map<number, Map<string, number>> = new Map();
  function clearErrorsAndWarnings() {
    
    for (const id of fiberIDToErrorsMap.keys()) {
      const fiber = idToArbitraryFiberMap.get(id);
      if (fiber != null) {
        fibersWithChangedErrorOrWarningCounts.add(fiber);
        updateMostRecentlyInspectedElementIfNecessary(id);
      }
    }
    
    for (const id of fiberIDToWarningsMap.keys()) {
      const fiber = idToArbitraryFiberMap.get(id);
      if (fiber != null) {
        fibersWithChangedErrorOrWarningCounts.add(fiber);
        updateMostRecentlyInspectedElementIfNecessary(id);
      }
    }
    fiberIDToErrorsMap.clear();
    fiberIDToWarningsMap.clear();
    flushPendingEvents();
  }
  function clearMessageCountHelper(