![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  
operations[i++] = key.length;
      const encodedKey = utfEncodeString(key);
      for (let j = 0; j < encodedKey.length; j++) {
        operations[i + j] = encodedKey[j];
      }
      i += key.length;
    });
    if (numUnmountIDs > 0) {
       A
      operations[i++] = TREE_OPERATION_REMOVE;
       T
      operations[i++] = numUnmountIDs;
       F
       T
       S
      for (let j = pendingRealUnmountedIDs.length - 1; j >= 0; j--) {
        operations[i++] = pendingRealUnmountedIDs[j];
      }
       Fill in the simulated unmounts (hidden Suspense subtrees) in their order.
       (We want children to go before parents.)
       They go *after* the real unmounts because we know for sure they won't be
       children of already pushed "real" IDs. If they were, we wouldn't be able
       to discover them during the traversal, as they would have been deleted.
      for (let j = 0; j < pendingSimulatedUnmountedIDs.length; j++) {
        operations[i + j] = pendingSimulatedUnmountedIDs[j];
      }
      i += pendingSimulatedUnmountedIDs.length;
       T
      if (pendingUnmountedRootID !== null) {
        operations[i] = pendingUnmountedRootID;