fix code completion when enableTrailingCommas is true

pkg/analysis_server/lib/src/provisional/completion/dart/completion_target.dart

 // Copyright (c) 2015, 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 analysis_server.src.provisional.completion.dart.completion_target;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/generated/utilities_dart.dart';
 
 int _computeArgIndex(AstNode containingNode, Object entity) {
   var argList = containingNode;
   if (argList is ArgumentList) {
     NodeList<Expression> args = argList.arguments;
     for (int index = 0; index < args.length; ++index) {
       if (entity == args[index]) {
         return index;
       }
     }
     if (args.isEmpty) {
       return 0;
     }
+    if (entity == argList.rightParenthesis) {
+      // Parser ignores trailing commas
+      if (argList.rightParenthesis.previous?.lexeme == ',') {
+        return args.length;
+      }
+      return args.length - 1;
+    }
   }
   return null;
 }
 
 /**
  * A CompletionTarget represents an edge in the parse tree which connects an
  * AST node (the [containingNode] of the completion) to one of its children
  * (the [entity], which represents the place in the parse tree where the newly
  * completed text will be inserted).
  *
@@ skipping 381 matching lines @@
       if (param.parameterKind == ParameterKind.NAMED) {
         // TODO(danrubel) handle named parameters
         return false;
       } else {
         return paramType is FunctionType || paramType is FunctionTypeAlias;
       }
     }
     return false;
   }
 }