There is an async Flask in the form of Sanic[1]. Looks quite usable compared to raw aiohttp or twisted spaghetti I had to help maintain a couple months ago.
Combined with aiohttp-json-rpc I had a very effective websocket-based JSON RPC backend service up and running in a very short time and with very little boilerplate. I'll be using this pattern again for sure.
What would have been the advantage of me using Sanic instead (I haven't RTFM for sanic yet but I'm about to...)?
Sanic also makes performance a key development goal so you can feel comfortable the project will scale gracefully. But I will stress I don't have current benchmarks to compare against aiohttp
With aiohttp you can make neat things it is operating a level below Sanic - in my experience people end up writing their own half-baked undocumented framework based on a combination of aiohttp, some ORM and a template engine - sometimes with half a dozen combinations in the same company.