![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  
function getUpdatersList(root): Array<SerializedElement> | null {
    return root.memoizedUpdaters != null
      ? Array.from(root.memoizedUpdaters).map(fiberToSerializedElement)
      : null;
  }
  function handleCommitFiberUnmount(fiber) {
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     W
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    recordUnmount(fiber, false);
  }
  function handlePostCommitFiberRoot(root) {
    if (isProfiling && rootSupportsProfiling(root)) {
      if (currentCommitProfilingMetadata !== null) {
        const {effectDuration, passiveEffectDuration} = getEffectDurations(
          root,
        );
        currentCommitProfilingMetadata.effectDuration = effectDuration;
        currentCommitProfilingMetadata.passiveEffectDuration = passiveEffectDuration;
      }
    }
  }
  function handleCommitFiberRoot(root, priorityLevel) {
    const current = root.current;
    const alternate = current.alternate;
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     I
    untrackFibers();
    currentRootID = getOrGenerateFiberID(current);
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