![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  
props: getChangedKeys(
              prevFiber.memoizedProps,
              nextFiber.memoizedProps,
            ),
            state: getChangedKeys(
              prevFiber.memoizedState,
              nextFiber.memoizedState,
            ),
          };
           O
          if (enableProfilerChangedHookIndices) {
            const indices = getChangedHooksIndices(
              prevFiber.memoizedState,
              nextFiber.memoizedState,
            );
            data.hooks = indices;
            data.didHooksChange = indices !== null && indices.length > 0;
          } else {
            data.didHooksChange = didHooksChange(
              prevFiber.memoizedState,
              nextFiber.memoizedState,
            );
          }
          return data;
        }
      default:
        return null;
    }
  }
  function updateContextsForFiber(fiber: Fiber) {