gistfile1.txt

    using System;
    using System.Collections.Generic;
    using System.Net.Http;
    using System.Threading.Tasks;
    using Xunit;

    public class Fixture
    {
        [Fact]
        public void Should_FactMethodName()
        {
            var module = new Class1();

            var result = module.routes["/"].Invoke(new object());

            var i = 10;
        }
    }

    public class Class1 : Module
    {
        public Class1()
        {
            Get["/"] = x =>
            {
                return "non async";
            };

            Get["/stuff"] = Async(async x =>
            {
                var client = new HttpClient();
                var result = await client.GetAsync("http://www.nancyfx.org");
                var content = await result.Content.ReadAsStringAsync();
                return content;
            });
        }
    }

    public class Module
    {
        public readonly IDictionary<string, Func<dynamic, dynamic>> routes = new Dictionary<string, Func<dynamic, dynamic>>();

        public Builder Get
        {
            get { return new Builder(this); }
        }

        // Helper to ensure that the async lambda is treated as a Task<dynamic>
        public static Func<dynamic, dynamic> Async(Func<dynamic, Task<dynamic>> function)
        {
            return function;
        }

        public class Builder
        {
            private readonly Module module;

            public Builder(Module module)
            {
                this.module = module;
            }

            //public Func<dynamic, dynamic> this[string path]
            //{
            //    set { this.module.routes.Add(path, value); }
            //}

            public Func<dynamic, Task<dynamic>> this[string path]
            {
                set { this.module.routes.Add(path, value); }
            }
        }
    }

Added an Async static method to allow for casting of async lambda to Func<dynamic, dynamic>