![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  
We might want to revisit this if it proves to be too inefficient.
    let child = childSet;
    while (child !== null) {
      findReorderedChildrenRecursively(child, nextChildren);
      child = child.sibling;
    }
    const numChildren = nextChildren.length;
    if (numChildren < 2) {
       N
      return;
    }
    pushOperation(TREE_OPERATION_REORDER_CHILDREN);
    pushOperation(getFiberIDThrows(fiber));
    pushOperation(numChildren);
    for (let i = 0; i < nextChildren.length; i++) {
      pushOperation(nextChildren[i]);
    }
  }
  function findReorderedChildrenRecursively(
    fiber: Fiber,
    nextChildren: Array<number>,
  ) {
    if (!shouldFilterFiber(fiber)) {
      nextChildren.push(getFiberIDThrows(fiber));
    } else {
      let child = fiber.child;
      const isTimedOutSuspense =
        fiber.tag === SuspenseComponent && fiber.memoizedState !== null;
      if (isTimedOutSuspense) {
         S