![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 message = format(...args);
    if (__DEBUG__) {
      debug('onErrorOrWarning', fiber, null, `${type}: "${message}"`);
    }
    
    fibersWithChangedErrorOrWarningCounts.add(fiber);
    
    const fiberMap =
      type === 'error' ? pendingFiberToErrorsMap : pendingFiberToWarningsMap;
    const messageMap = fiberMap.get(fiber);
    if (messageMap != null) {
      const count = messageMap.get(message) || 0;
      messageMap.set(message, count + 1);
    } else {
      fiberMap.set(fiber, new Map([[message, 1]]));
    }
    
    
    
    
    
    
    flushPendingErrorsAndWarningsAfterDelay();
  }
  
  
  
  
  
  if (process.env.NODE_ENV !== 'test') {