![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  
PathMatch,
  ProfilingDataBackend,
  ProfilingDataForRootBackend,
  ReactRenderer,
  RendererInterface,
  SerializedElement,
  WorkTagMap,
} from './types';
import type {
  ComponentFilter,
  ElementType,
} from 'react-devtools-shared/src/types';
type getDisplayNameForFiberType = (fiber: Fiber) => string | null;
type getTypeSymbolType = (type: any) => Symbol | number;
type ReactPriorityLevelsType = {|
  ImmediatePriority: number,
  UserBlockingPriority: number,
  NormalPriority: number,
  LowPriority: number,
  IdlePriority: number,
  NoPriority: number,
|};
type ReactTypeOfSideEffectType = {|
  DidCapture: number,
  NoFlags: number,
  PerformedWork: number,
  Placement: number,
  Incomplete: number,
|};
function getFiberFlags(fiber: Fiber): number {