Initial release, with CancelToken and Cancel from the November 23rd, 2016 draft of https://tc39.github.io/proposal-cancelable-promises/ (now withdrawn).
Looks like https://github.com/littledan/proposal-cancelable-promises may be picking up the spec and removing the syntactic parts, leaving just the polyfillable pieces implemented here.
Also includes two non-spec'd but useful methods: isCancelToken and isCancel.