![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 elementType = getElementTypeForFiber(fiber);
    if (hideElementsWithTypes.has(elementType)) {
      return true;
    }
    if (hideElementsWithDisplayNames.size > 0) {
      const displayName = getDisplayNameForFiber(fiber);
      if (displayName != null) {
        
        for (const displayNameRegExp of hideElementsWithDisplayNames) {
          if (displayNameRegExp.test(displayName)) {
            return true;
          }
        }
      }
    }
    if (_debugSource != null && hideElementsWithPaths.size > 0) {
      const {fileName} = _debugSource;
      
      for (const pathRegExp of hideElementsWithPaths) {
        if (pathRegExp.test(fileName)) {
          return true;
        }
      }
    }
    return false;
  }
  
  function getElementTypeForFiber(fiber: Fiber): ElementType {
    const {type, tag} = fiber;
    switch (tag) {