Seriously. I've wanted something like this in javascript for ages. Given the complete async nature of modern javascript, it would be a huge win. On the other hand, async code in javascript (as it is now) does look cleaner than the equivalent on C#.
Maybe you'll find stratified js -- http://stratifiedjs.org/ -- interesting. I haven't tried it myself, but it seems to fit the bill. As a side note: IMHO C#'s use of only 2 keywords and method combinators is cleaner than stratified js' approach.