![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  
pushOperation(TREE_OPERATION_UPDATE_ERRORS_OR_WARNINGS);
        pushOperation(fiberID);
        pushOperation(errorCount);
        pushOperation(warningCount);
      }
       A
      pendingFiberToErrorsMap.delete(fiber);
      pendingFiberToWarningsMap.delete(fiber);
    });
    fibersWithChangedErrorOrWarningCounts.clear();
  }
  function flushPendingEvents(root: Object): void {
     A
     W
    recordPendingErrorsAndWarnings();
    if (
      pendingOperations.length === 0 &&
      pendingRealUnmountedIDs.length === 0 &&
      pendingSimulatedUnmountedIDs.length === 0 &&
      pendingUnmountedRootID === null
    ) {
       I
       N
      
       T
       (
       (
       B
       S
      if (!isProfiling) {