Remove VisualStudioSourceInformationProvider and DiaSession-related c…

…lasses

src/common/NullMessageSink.cs

@@ -17,6 +17,11 @@ public class NullMessageSink : LongLivedMarshalByRefObject, IMessageSink
         , IMessageSinkWithTypes
 #endif
     {
+        /// <summary>
+        /// Gets the singleton instance of the <see cref="NullMessageSink"/>.
+        /// </summary>
+        public static NullMessageSink Instance { get; } = new NullMessageSink();
+
         /// <inheritdoc/>
         public void Dispose() { }
 

src/xunit.runner.utility/Frameworks/NullSourceInformationProvider.cs

@@ -10,6 +10,11 @@ namespace Xunit
     /// </summary>
     public class NullSourceInformationProvider : LongLivedMarshalByRefObject, ISourceInformationProvider
     {
+        /// <summary>
+        /// Gets the singleton instance of the <see cref="NullSourceInformationProvider"/>.
+        /// </summary>
+        public static NullSourceInformationProvider Instance { get; } = new NullSourceInformationProvider();
+
         /// <inheritdoc/>
         public ISourceInformation GetSourceInformation(ITestCase testCase)
         {

src/xunit.runner.utility/Frameworks/XunitFrontController.cs

@@ -57,21 +57,10 @@ public class XunitFrontController : IFrontController, ITestCaseDescriptorProvide
             this.configFileName = configFileName;
             this.shadowCopy = shadowCopy;
             this.shadowCopyFolder = shadowCopyFolder;
-            this.sourceInformationProvider = sourceInformationProvider;
-            this.diagnosticMessageSink = diagnosticMessageSink ?? new NullMessageSink();
+            this.sourceInformationProvider = sourceInformationProvider ?? NullSourceInformationProvider.Instance;
+            this.diagnosticMessageSink = diagnosticMessageSink ?? NullMessageSink.Instance;
 
             Guard.FileExists("assemblyFileName", assemblyFileName);
-
-            if (this.sourceInformationProvider == null)
-            {
-#if NETSTANDARD
-                this.sourceInformationProvider = new NullSourceInformationProvider();
-#else
-                this.sourceInformationProvider = new VisualStudioSourceInformationProvider(assemblyFileName);
-#endif
-                toDispose.Push(this.sourceInformationProvider);
-            }
-
         }
 
         ITestCaseBulkDeserializer BulkDeserializer