Fix following comma detection in code completion (#29503).

pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart

Index: pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart b/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart
index 01775a3221b3737590a19959907318177ba623e3..59fc1c0e8292a773c0a752bf681e9952601eca12 100644
--- a/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart
+++ b/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart
@@ -330,10 +330,17 @@ class ArgListContributor extends DartCompletionContributor {
   }
 
   bool _isFollowedByAComma(DartCompletionRequest request) {
+    // new A(^); NO
+    // new A(one: 1, ^); NO
+    // new A(^ , one: 1); YES
+    // new A(^), ... NO
+
+    var containingNode = request.target.containingNode;
     var entity = request.target.entity;
     Token token =
         entity is AstNode ? entity.endToken : entity is Token ? entity : null;
-    return token?.next?.type == TokenType.COMMA;
+    return (token != containingNode?.endToken) &&
+        token?.next?.type == TokenType.COMMA;
   }
 
   bool _isInFlutterCreation(DartCompletionRequest request) {