As of the date of this comment, promises aren't flattened, only thenables. `Promise.isPromise(x)` returns false positives for unknown thenables, so this treats promises and thenables identically, which doesn't satisfy 1.1.

I'm open to suggestions on how to reconcile the issue. IMO, `Promise.isPromise(x)` should return `true` for instances of Y.Promise from another YUI instance or another window.