![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  
* LICENSE file in the root directory of this source tree.
 *
 * @flow
 */
import {gt, gte} from 'semver';
import {
  ComponentFilterDisplayName,
  ComponentFilterElementType,
  ComponentFilterHOC,
  ComponentFilterLocation,
  ElementTypeClass,
  ElementTypeContext,
  ElementTypeFunction,
  ElementTypeForwardRef,
  ElementTypeHostComponent,
  ElementTypeMemo,
  ElementTypeOtherOrUnknown,
  ElementTypeProfiler,
  ElementTypeRoot,
  ElementTypeSuspense,
  ElementTypeSuspenseList,
} from 'react-devtools-shared/src/types';
import {
  deletePathInObject,
  getDisplayName,
  getDefaultComponentFilters,
  getInObject,
  getUID,
  renamePathInObject,
  setInObject,