![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  
DEPRECATED_ASYNC_MODE_SYMBOL_STRING,
  PROVIDER_NUMBER,
  PROVIDER_SYMBOL_STRING,
  CONTEXT_NUMBER,
  CONTEXT_SYMBOL_STRING,
  STRICT_MODE_NUMBER,
  STRICT_MODE_SYMBOL_STRING,
  PROFILER_NUMBER,
  PROFILER_SYMBOL_STRING,
  SCOPE_NUMBER,
  SCOPE_SYMBOL_STRING,
  FORWARD_REF_NUMBER,
  FORWARD_REF_SYMBOL_STRING,
  MEMO_NUMBER,
  MEMO_SYMBOL_STRING,
} from './ReactSymbols';
import {format} from './utils';
import {enableProfilerChangedHookIndices} from 'react-devtools-feature-flags';
import is from 'shared/objectIs';
import isArray from 'shared/isArray';
import type {Fiber} from 'react-reconciler/src/ReactInternalTypes';
import type {
  ChangeDescription,
  CommitDataBackend,
  DevToolsHook,
  InspectedElement,
  InspectedElementPayload,
  InstanceAndStyle,
  NativeType,
  PathFrame,