![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  
case ClassComponent:
      case IncompleteClassComponent:
        return ElementTypeClass;
      case FunctionComponent:
      case IndeterminateComponent:
        return ElementTypeFunction;
      case ForwardRef:
        return ElementTypeForwardRef;
      case HostRoot:
        return ElementTypeRoot;
      case HostComponent:
        return ElementTypeHostComponent;
      case HostPortal:
      case HostText:
      case Fragment:
        return ElementTypeOtherOrUnknown;
      case MemoComponent:
      case SimpleMemoComponent:
        return ElementTypeMemo;
      case SuspenseComponent:
        return ElementTypeSuspense;
      case SuspenseListComponent:
        return ElementTypeSuspenseList;
      default:
        const typeSymbol = getTypeSymbol(type);
        switch (typeSymbol) {
          case CONCURRENT_MODE_NUMBER:
          case CONCURRENT_MODE_SYMBOL_STRING:
          case DEPRECATED_ASYNC_MODE_SYMBOL_STRING:
            return ElementTypeOtherOrUnknown;