Add IAsyncDisposable.

Ix.NET/Source/System.Interactive.Async/IAsyncDisposable.cs

@@ -0,0 +1,13 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information. 
+
+using System.Threading.Tasks;
+
+namespace System
+{
+    public interface IAsyncDisposable
+    {
+        Task DisposeAsync();
+    }
+}