Fix RPS regression due to overly aggressive assertions.

src/EditorFeatures/Core/Shared/Utilities/ForegroundThreadAffinitizedObject.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using System;
+using System.Diagnostics;
 using System.Threading;
 using System.Threading.Tasks;
 using System.Windows;
@@ -109,12 +110,18 @@ public void AssertIsForeground()
         {
             var whenCreatedThread = _foregroundThreadDataWhenCreated.Thread;
             var currentThread = Thread.CurrentThread;
-            Contract.ThrowIfFalse(currentThread == whenCreatedThread, 
+
+            // In debug, provide a lot more information so that we can track down unit test flakeyness.
+            // This is too expensive to do in retail as it creates way too many allocations.
+            Debug.Assert(currentThread == whenCreatedThread,
                 "When created kind       : " + _foregroundThreadDataWhenCreated.Kind + "\r\n" +
                 "When created thread id  : " + whenCreatedThread?.ManagedThreadId + "\r\n" +
                 "When created thread name: " + whenCreatedThread?.Name + "\r\n" +
-                "Current thread id       : " + currentThread?.ManagedThreadId + "\r\n" + 
+                "Current thread id       : " + currentThread?.ManagedThreadId + "\r\n" +
                 "Current thread name     : " + currentThread?.Name);
+
+            // But, in retail, do the check as well, so that we can catch problems that happen in the wild.
+            Contract.ThrowIfFalse(currentThread == whenCreatedThread);
         }
 
         public void AssertIsBackground()