non-self referencial cyclical promises?

[Benjamin Gruenbaum]

For what it's worth, bluebird promises detect the error and reject with:

```
TypeError: Chaining cycle detected for promise #<Promise>
```

So it's both possible and not a performance issue. For this case, a WeakMap
is not needed for this case. https://jsfiddle.net/41ez2b6d/ .


> We ran into code "in the wild"

Yes, while I've never run into this I've talked to several people who have
ran into this error - so it's definitely "a thing".
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