![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  
switch (tag) {
      case DehydratedSuspenseComponent:
         T
         H
         a
         F
         h
        return true;
      case HostPortal:
      case HostText:
      case Fragment:
      case LegacyHiddenComponent:
      case OffscreenComponent:
        return true;
      case HostRoot:
         I
        return false;
      default:
        const typeSymbol = getTypeSymbol(type);
        switch (typeSymbol) {
          case CONCURRENT_MODE_NUMBER:
          case CONCURRENT_MODE_SYMBOL_STRING:
          case DEPRECATED_ASYNC_MODE_SYMBOL_STRING:
          case STRICT_MODE_NUMBER:
          case STRICT_MODE_SYMBOL_STRING:
            return true;
          default:
            break;
        }
    }