Can Flask use the async feature of Tornado Server?
No. It is possible to run Flask on Tornado's WSGIContainer, but since Flask is limited by the WSGI interface it will be unable to take advantage of Tornado's asynchronous features. gunicorn or uwsgi is generally a much better choice than Tornado's WSGIContainer unless you have a specific need to run a Flask application in the same process as native Tornado RequestHandlers.
We have a project use Flask+Gunicorn(sync). This works well for a long time,  however, recently i came across to know that Asyncio(Python3.5) support async io in standard library. 
However, before Asyncio, there are both Twisted and Tornado async servers. So, i wander whether Flask can use the aysncio feature of Tornado, cause Gunicorn support tornado worker class.