Add Shared attributes to code fix providers

src/Features/CSharp/Portable/UseIsNullCheck/CSharpUseIsNullCheckCodeFixProvider.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.Composition;
 using Microsoft.CodeAnalysis.CodeFixes;
 using Microsoft.CodeAnalysis.CSharp.Extensions;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
@@ -7,7 +8,7 @@
 
 namespace Microsoft.CodeAnalysis.CSharp.UseIsNullCheck
 {
-    [ExportCodeFixProvider(LanguageNames.CSharp)]
+    [ExportCodeFixProvider(LanguageNames.CSharp), Shared]
     internal class CSharpUseIsNullCheckCodeFixProvider : AbstractUseIsNullCheckCodeFixProvider
     {
         protected override string GetIsNullTitle()

src/Features/VisualBasic/Portable/UseIsNullCheck/VisualBasicUseIsNullCheckCodeFixProvider.vb

 ' Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+Imports System.Composition
 Imports Microsoft.CodeAnalysis.CodeFixes
 Imports Microsoft.CodeAnalysis.VisualBasic.Syntax
 Imports Microsoft.CodeAnalysis.UseIsNullCheck
 
 Namespace Microsoft.CodeAnalysis.VisualBasic.UseIsNullCheck
-    <ExportCodeFixProvider(LanguageNames.VisualBasic)>
+    <ExportCodeFixProvider(LanguageNames.VisualBasic), [Shared]>
     Friend Class VisualBasicUseIsNullCheckCodeFixProvider
         Inherits AbstractUseIsNullCheckCodeFixProvider