Adding IClock.

AsyncRx.NET/System.Reactive.Async/System/Reactive/Concurrency/IClock.cs

@@ -0,0 +1,11 @@
+// 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. 
+
+namespace System.Reactive.Concurrency
+{
+    public interface IClock
+    {
+        DateTimeOffset Now { get; }
+    }
+}