Mark new contract an experimental feature.

Author: Lifeng Lu

src/Microsoft.VisualStudio.Threading/IPendingExecutionRequestState.cs

@@ -1,11 +1,16 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
+using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
+
 namespace Microsoft.VisualStudio.Threading
 {
     /// <summary>
     /// An optional interface implemented by pending request state posted to the underline synchronization context. It allows synchronization context to remove completed requests.
     /// </summary>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    [Experimental("VSOnly")]
     public interface IPendingExecutionRequestState
     {
         /// <summary>

src/Microsoft.VisualStudio.Threading/JoinableTaskFactory.cs

@@ -1058,7 +1058,9 @@ public void Dispose()
     /// A delegate wrapper that ensures the delegate is only invoked at most once.
     /// </summary>
     [DebuggerDisplay("{DelegateLabel}")]
+#pragma warning disable VSOnly // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
     internal class SingleExecuteProtector : IPendingExecutionRequestState
+#pragma warning restore VSOnly // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
     {
         /// <summary>
         /// Executes the delegate if it has not already executed.