Change product code to work with editor expectations.

- Descriptor providers should not be pushing `null` descriptors into the overall list of `TagHelperDescriptor`s; this causes null refs on the editor side of things at design time.
- Expose `FilePath` on `RazorEditorParser` for the editor. The editor currenlty uses the `RazorEditorParser` as a place to hold some state about the Razor document.

src/Microsoft.AspNetCore.Mvc.Razor.Extensions/ViewComponentTagHelperDescriptorProvider.cs

@@ -55,7 +55,12 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
             var factory = new ViewComponentTagHelperDescriptorFactory(compilation);
             for (var i = 0; i < types.Count; i++)
             {
-                context.Results.Add(factory.CreateDescriptor(types[i]));
+                var descriptor = factory.CreateDescriptor(types[i]);
+
+                if (descriptor != null)
+                {
+                    context.Results.Add(descriptor);
+                }
             }
         }
 

src/Microsoft.CodeAnalysis.Razor/DefaultTagHelperDescriptorProvider.cs

@@ -48,7 +48,11 @@ namespace Microsoft.CodeAnalysis.Razor
             for (var i = 0; i < types.Count; i++)
             {
                 var descriptor = factory.CreateDescriptor(types[i]);
-                context.Results.Add(descriptor);
+
+                if (descriptor != null)
+                {
+                    context.Results.Add(descriptor);
+                }
             }
         }
 

src/Microsoft.VisualStudio.LanguageServices.Razor/RazorEditorParser.cs

@@ -30,6 +30,7 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor
             }
 
             TemplateEngine = templateEngine;
+            FilePath = filePath;
             _parser = new BackgroundParser(templateEngine, filePath);
             _parser.ResultsReady += (sender, args) => OnDocumentParseComplete(args);
             _parser.Start();
@@ -42,6 +43,8 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor
 
         public RazorTemplateEngine TemplateEngine { get; }
 
+        public string FilePath { get; }
+
         // Internal for testing.
         internal RazorSyntaxTree CurrentSyntaxTree { get; private set; }
 

test/Microsoft.CodeAnalysis.Razor.Test/DefaultTagHelperDescriptorProviderTest.cs

@@ -0,0 +1,38 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Linq;
+using System.Reflection;
+using Microsoft.AspNetCore.Razor.Language;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.Razor
+{
+    public class DefaultTagHelperDescriptorProviderTest
+    {
+        private static readonly Assembly _assembly = typeof(DefaultTagHelperDescriptorProviderTest).GetTypeInfo().Assembly;
+
+        [Fact]
+        public void Execute_DoesNotAddEditorBrowsableNeverDescriptorsAtDesignTime()
+        {
+            // Arrange
+            var editorBrowsableTypeName = "Microsoft.CodeAnalysis.Razor.Workspaces.Test.EditorBrowsableTagHelper";
+            var compilation = TestCompilation.Create(_assembly);
+            var descriptorProvider = new DefaultTagHelperDescriptorProvider()
+            {
+                DesignTime = true,
+            };
+            var context = TagHelperDescriptorProviderContext.Create();
+
+            // Act 
+            descriptorProvider.Execute(context);
+
+            // Assert
+            Assert.NotNull(compilation.GetTypeByMetadataName(editorBrowsableTypeName));
+            var nullDescriptors = context.Results.Where(descriptor => descriptor == null);
+            Assert.Empty(nullDescriptors);
+            var editorBrowsableDescriptor = context.Results.Where(descriptor => descriptor.GetTypeName() == editorBrowsableTypeName);
+            Assert.Empty(editorBrowsableDescriptor);
+        }
+    }
+}