i.e. if you want port you program to dotty without changing source code, then you can replace scala-async dependecy to this shim for building with dotty.
libraryDependencies += "com.github.rssh" %% "shim-scala-async-dotty-cps-async" % "0.9.4",
['--' in github project name changed to '-' ]
Note, that if you want bug-to-bug compatibility with scala2, than you probably want wait for official port of scala-async compiler plugin.