Seal a few more extensions types with only a private ctor

src/libraries/Microsoft.Extensions.FileProviders.Abstractions/ref/Microsoft.Extensions.FileProviders.Abstractions.cs

@@ -46,7 +46,7 @@ public NotFoundFileInfo(string name) { }
         [System.Diagnostics.CodeAnalysis.DoesNotReturn]
         public System.IO.Stream CreateReadStream() { throw null; }
     }
-    public partial class NullChangeToken : Microsoft.Extensions.Primitives.IChangeToken
+    public sealed partial class NullChangeToken : Microsoft.Extensions.Primitives.IChangeToken
     {
         internal NullChangeToken() { }
         public bool ActiveChangeCallbacks { get { throw null; } }

src/libraries/Microsoft.Extensions.FileProviders.Abstractions/src/NullChangeToken.cs

@@ -9,7 +9,7 @@ namespace Microsoft.Extensions.FileProviders
     /// <summary>
     /// An empty change token that doesn't raise any change callbacks.
     /// </summary>
-    public class NullChangeToken : IChangeToken
+    public sealed class NullChangeToken : IChangeToken
     {
         /// <summary>
         /// Gets a singleton instance of <see cref="NullChangeToken"/>.

src/libraries/Microsoft.Extensions.Logging.Abstractions/ref/Microsoft.Extensions.Logging.Abstractions.cs

@@ -193,7 +193,7 @@ public void AddProvider(Microsoft.Extensions.Logging.ILoggerProvider provider) {
         public Microsoft.Extensions.Logging.ILogger CreateLogger(string name) { throw null; }
         public void Dispose() { }
     }
-    public partial class NullLoggerProvider : Microsoft.Extensions.Logging.ILoggerProvider, System.IDisposable
+    public sealed partial class NullLoggerProvider : Microsoft.Extensions.Logging.ILoggerProvider, System.IDisposable
     {
         internal NullLoggerProvider() { }
         public static Microsoft.Extensions.Logging.Abstractions.NullLoggerProvider Instance { get { throw null; } }

src/libraries/Microsoft.Extensions.Logging.Abstractions/src/NullLoggerProvider.cs

@@ -6,7 +6,7 @@ namespace Microsoft.Extensions.Logging.Abstractions
     /// <summary>
     /// Provider for the <see cref="NullLogger"/>.
     /// </summary>
-    public class NullLoggerProvider : ILoggerProvider
+    public sealed class NullLoggerProvider : ILoggerProvider
     {
         /// <summary>
         /// Returns an instance of <see cref="NullLoggerProvider"/>.