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

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library services.completion.contributor.dart.arglist;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/ide_options.dart';
 import 'package:analysis_server/src/protocol_server.dart'
@@ skipping 312 matching lines @@
         if (isFlutterWidget(element.enclosingElement) &&
             param.name == 'children') {
           return getDefaultStringParameterValue(param);
         }
       }
     }
     return null;
   }
 
   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) {
     AstNode containingNode = request?.target?.containingNode;
     InstanceCreationExpression newExpr = containingNode != null
         ? identifyNewExpression(containingNode.parent)
         : null;
     return newExpr != null && isFlutterInstanceCreationExpression(newExpr);
   }
 
   /**
    * If the given [comment] is not `null`, fill the [suggestion] documentation
    * fields.
    */
   static void _setDocumentation(
       CompletionSuggestion suggestion, String comment) {
     if (comment != null) {
       String doc = removeDartDocDelimiters(comment);
       suggestion.docComplete = doc;
       suggestion.docSummary = getDartDocSummary(doc);
     }
   }
 }