How to Make ASP.NET MVC Recognize IHttpAsyncHandler from IRouteHandler.GetHttpHandler()?

public class AsyncMvcRouteHandler : IRouteHandler
{
    public IHttpHandler GetHttpHandler(RequestContext requestContext)
    {
        return new AsyncMvcHandler(requestContext);
    }

    class AsyncMvcHandler : IHttpAsyncHandler, IRequiresSessionState
    {
        public AsyncMvcHandler(RequestContext context)
        {
        }

        // IHttpHandler members
        public bool IsReusable { get { return false; } }
        public void ProcessRequest(HttpContext httpContext) { throw new NotImplementedException(); }

        // IHttpAsyncHandler members
        public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
        {
            throw new NotImplementedException();
        }

        public void EndProcessRequest(IAsyncResult result)
        {
            throw new NotImplementedException();
        }
    }
}

routes.Add(new Route("{controller}/{action}/{id}", new AsyncMvcRouteHandler())
        {
            Defaults = new RouteValueDictionary(new { controller = "Home", action = "Index", id = "" }),
        });
    }

routes.MapRoute(
    "Default",
    "{controller}/{action}",
    new { controller = "Home", action = "Index" }
);