Async/await is specialised syntax for a more generic concept of monads which haskell was the language in which this was heavily researched.

Specifically continuation monad describes asynchronous computation (promises) and was one of the motivating examples In the early 90s going back all the way to Moggis original "notions of computations" paper from 1991 http://homepages.inf.ed.ac.uk/wadler/topics/monads.html

Async/await and LINQ were the brainchild of Erik Meijer. A haskell researcher.

Also see https://news.ycombinator.com/item?id=25565229 from a recent discussion here.

Specifically http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.72.... where Eric tells his story about bringing haskell concepts to .NET