![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  
if (isTimedOut) {
           S
           g
           i
          const primaryChildFragment = fiber.child;
          const fallbackChildFragment = primaryChildFragment
            ? primaryChildFragment.sibling
            : null;
          const fallbackChild = fallbackChildFragment
            ? fallbackChildFragment.child
            : null;
          if (fallbackChild !== null) {
            mountFiberRecursively(
              fallbackChild,
              shouldIncludeInTree ? fiber : parentFiber,
              true,
              traceNearestHostComponentUpdate,
            );
          }
        } else {
          let primaryChild: Fiber | null = null;
          const areSuspenseChildrenConditionallyWrapped =
            OffscreenComponent === -1;
          if (areSuspenseChildrenConditionallyWrapped) {
            primaryChild = fiber.child;
          } else if (fiber.child !== null) {
            primaryChild = fiber.child.child;
          }
          if (primaryChild !== null) {
            mountFiberRecursively(