Fix for assertion error

src/EditorFeatures/Core/FindUsages/AbstractFindUsagesService_FindReferences.cs

@@ -20,7 +20,7 @@ namespace Microsoft.CodeAnalysis.Editor.FindUsages
 {
     internal abstract partial class AbstractFindUsagesService
     {
-        public async Task FindReferencesAsync(
+        async Task IFindUsagesService.FindReferencesAsync(
             Document document, int position, IFindUsagesContext context)
         {
             var definitionTrackingContext = new DefinitionTrackingContext(context);
@@ -48,6 +48,18 @@ internal abstract partial class AbstractFindUsagesService
             }
         }
 
+        async Task IFindUsagesLSPService.FindReferencesAsync(
+            Document document, int position, IFindUsagesContext context)
+        {
+            // We don't need to get third party definitions when finding references in LSP.
+            // Currently, 3rd party definitions = XAML definitions, and XAML will provide
+            // references via LSP instead of hooking into Roslyn.
+            // This also means that we don't need to be on the UI thread.
+            var definitionTrackingContext = new DefinitionTrackingContext(context);
+            await FindLiteralOrSymbolReferencesAsync(
+                document, position, definitionTrackingContext).ConfigureAwait(false);
+        }
+
         private async Task FindLiteralOrSymbolReferencesAsync(
             Document document, int position, IFindUsagesContext context)
         {

src/Features/LanguageServer/Protocol/CustomProtocol/FindUsagesLSPContext.cs

@@ -12,7 +12,6 @@
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.Classification;
-using Microsoft.CodeAnalysis.Editor;
 using Microsoft.CodeAnalysis.ErrorReporting;
 using Microsoft.CodeAnalysis.FindSymbols;
 using Microsoft.CodeAnalysis.FindSymbols.Finders;

src/Features/LanguageServer/Protocol/Handler/References/FindAllReferencesHandler.cs

@@ -9,7 +9,6 @@
 using System.Diagnostics;
 using System.Threading;
 using System.Threading.Tasks;
-using Microsoft.CodeAnalysis.Editor;
 using Microsoft.CodeAnalysis.Editor.FindUsages;
 using Microsoft.CodeAnalysis.Host.Mef;
 using Microsoft.CodeAnalysis.LanguageServer.CustomProtocol;