I read somewhere in a pull request, on the Google Group, or in a defect that we as a Spine community were kind of undecided when it comes to a preferred Promise implementation (but I don't remember where I got into that discussion). I mentioned at the time that both jQuery and Zepto have Promise implementations (although the one in jQuery is hardly up to "spec").

What I've done here is opted to code against the implementation supposedly implemented by Chrome and Opera as "the ES6 implementation" and just provided a polyfill that bridges the interface to a jQuery Promise. Maybe now's a good time to pick a Promise implementation and run with it?