![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  
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        forceErrorForFiberIDs.delete(id);
        if (forceErrorForFiberIDs.size === 0) {
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          setErrorHandler(shouldErrorFiberAlwaysNull);
        }
      }
    }
    return status;
  }
  function overrideError(id, forceError) {
    if (
      typeof setErrorHandler !== 'function' ||
      typeof scheduleUpdate !== 'function'
    ) {
      throw new Error(
        'Expected overrideError() to not get called for earlier React versions.',
      );
    }
    forceErrorForFiberIDs.set(id, forceError);
    if (forceErrorForFiberIDs.size === 1) {
       F