See https://github.com/whatwg/fetch/issues/20 for some discussion on the matter.

I have library there I made ajax abtraction which uses native Promise and returns them. Without ability to abort such requests it's a pain developing normal apps which uses ajax. So I decided to make some sort of subclassing of Promise, so ajax returns promise with additional method "abort". Of course, this cannot help in all situations such as Promise.all(...), but at least it works in situations such this:

var req = ajax(...).then(funnction() {return new Promise(...)});
...
req.abort(); // subclass of promise, actually native promise monkey-patched with |abort| method
...

https://github.com/slightlyoff/ServiceWorker/issues/625 is now tracking this.