I'm thinking that maybe it makes sense to randomize this key to ensure that different copies of React don't conflict. this should also solve your test problem requiring the deletion of document._reactTopListenersID on every test. something like

```
var topListenersIDKey = "_reactTopListenersID" + String(Math.random()).slice(2);
```

which is essentially what jQuery does for a similar problem: https://github.com/jquery/jquery/blob/master/src/core.js#L193-L194