The Promises spec currently has API hooks for fulfilling and rejecting a promise.  It should also have a hook for adopting a promise.

Use-case: I'm returning a promise from an operation in the Font Load Events API.  Sometimes it'll be rejected quickly and directly, so I need to have a handle for the promise itself.  If all goes well, though, the result should be as if Promise.every() was called on a spec-defined list of other promises, which requires me to have the original returned promise adopt the result of the Promise.every() algorithm (once it's defined in a spec fashion that can be safely invoked).