Clean up C# extern alias completion provider

src/Features/CSharp/Portable/Completion/CompletionProviders/ExternAliasCompletionProvider.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.Collections.Generic;
 using System.Linq;
-using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.Completion;
-using Microsoft.CodeAnalysis.Completion.Providers;
 using Microsoft.CodeAnalysis.CSharp.Extensions;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.Options;
@@ -15,23 +12,30 @@
 
 namespace Microsoft.CodeAnalysis.CSharp.Completion.Providers
 {
-    internal class ExternAliasCompletionProvider : AbstractCompletionProvider
+    internal class ExternAliasCompletionProvider : CompletionListProvider
     {
         public override bool IsTriggerCharacter(SourceText text, int characterPosition, OptionSet options)
         {
             return CompletionUtilities.IsTriggerCharacter(text, characterPosition, options);
         }
 
-        protected override async Task<IEnumerable<CompletionItem>> GetItemsWorkerAsync(Document document, int position, CompletionTriggerInfo triggerInfo, CancellationToken cancellationToken)
+        public override async Task ProduceCompletionListAsync(CompletionListContext context)
         {
+            var document = context.Document;
+            var position = context.Position;
+            var cancellationToken = context.CancellationToken;
+
             var tree = await document.GetSyntaxTreeAsync(cancellationToken).ConfigureAwait(false);
 
             if (tree.IsInNonUserCode(position, cancellationToken))
             {
-                return null;
+                return;
             }
 
-            var targetToken = tree.FindTokenOnLeftOfPosition(position, cancellationToken).GetPreviousTokenIfTouchingWord(position);
+            var targetToken = tree
+                .FindTokenOnLeftOfPosition(position, cancellationToken)
+                .GetPreviousTokenIfTouchingWord(position);
+
             if (targetToken.IsKind(SyntaxKind.AliasKeyword) && targetToken.Parent.IsKind(SyntaxKind.ExternAliasDirective))
             {
                 var compilation = await document.Project.GetCompilationAsync(cancellationToken).ConfigureAwait(false);
@@ -40,16 +44,22 @@ protected override async Task<IEnumerable<CompletionItem>> GetItemsWorkerAsync(D
                 if (aliases.Any())
                 {
                     var root = await tree.GetRootAsync(cancellationToken).ConfigureAwait(false);
-                    var usedAliases = root.ChildNodes().OfType<ExternAliasDirectiveSyntax>().Where(e => !e.Identifier.IsMissing).Select(e => e.Identifier.ValueText);
+                    var usedAliases = root.ChildNodes().OfType<ExternAliasDirectiveSyntax>()
+                        .Where(e => !e.Identifier.IsMissing)
+                        .Select(e => e.Identifier.ValueText);
+
                     aliases.RemoveRange(usedAliases);
                     aliases.Remove(MetadataReferenceProperties.GlobalAlias);
-                    var textChangeSpan = CompletionUtilities.GetTextChangeSpan(await document.GetTextAsync(cancellationToken).ConfigureAwait(false), position);
-                    return aliases.Select(e =>
-                        new CompletionItem(this, e, textChangeSpan, glyph: Glyph.Namespace));
+
+                    var text = await document.GetTextAsync(cancellationToken).ConfigureAwait(false);
+                    var filterSpan = CompletionUtilities.GetTextChangeSpan(text, position);
+
+                    foreach (var alias in aliases)
+                    {
+                        context.AddItem(new CompletionItem(this, alias, filterSpan, glyph: Glyph.Namespace));
+                    }
                 }
             }
-
-            return SpecializedCollections.EmptyEnumerable<CompletionItem>();
         }
     }
 }