I don't think async is currently in the official spec. Last time I checked it was stage 3.
It certainly takes its time :) Well, Bublé sound cool for projects where feature stability over time is critical (like huge codebases), but I will still prefer to go with babel to select which stage I want.
EDIT : just to be clear, I won't advice any sized team to go with staged features. It works for me because I do a lot a small sized project working alone on them and I'm ok refactoring their codebase if a feature implementation change.
It really does cleanup a lot of code, but getting people to understand that an Async function returns a promise isn't always the easiest thing to convey until after it's been used. I resisted Promises for a while, once it became a part of ES6, and with async/await in Babel, I started using it... the cleaner code is worth the minor bit of overhead.