Implement async/await using Fiber

[Yuxuan Shui via Digitalmars-d-learn]

I find this difficult because I can't passing data via 
Fiber.yield/Fiber.call pair. e.g. I want something like:

void fiberFunc() {
    //Add some file descriptor to main loop
    string y = Fiber.yield();
    writeln(y);
}

auto f = new Fiber(&fiberFunc);
f.call();
mainloop {
    if (fd_readable)
       f.call(fd.rawRead());
}

I can probably using a derived fiber class and pass the result 
via class members, but that seems clumsy and not very general. 
Any ideas?