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

pkg/analysis_server/test/services/completion/dart/arglist_contributor_test.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 test.services.completion.dart.arglist;
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/ide_options.dart';
 import 'package:analysis_server/src/provisional/completion/dart/completion_dart.dart';
 import 'package:analysis_server/src/services/completion/dart/arglist_contributor.dart';
@@ skipping 874 matching lines @@
     // Don't generate children boilerplate.
     await computeSuggestions();
 
     assertSuggest('children: ,',
         csKind: CompletionSuggestionKind.NAMED_ARGUMENT,
         relevance: DART_RELEVANCE_NAMED_PARAMETER,
         defaultArgListString: null, // No default values.
         selectionOffset: 10);
   }
 
+  test_ArgumentList_Flutter_InstanceCreationExpression_01() async {
+    configureFlutterPkg({
+      'src/widgets/framework.dart': flutter_framework_code,
+    });
+
+    addTestSource('''
+import 'package:flutter/src/widgets/framework.dart';
+
+  build() => new Scaffold(
+        appBar: new AppBar(
+          ^
+        ),
+  );
+''');
+
+    // Don't generate children boilerplate.
+    await computeSuggestions();
+
+    assertSuggest('color: ,',
+        csKind: CompletionSuggestionKind.NAMED_ARGUMENT,
+        relevance: DART_RELEVANCE_NAMED_PARAMETER,
+        defaultArgListString: null, // No default values.
+        selectionOffset: 7);
+  }
+
   test_ArgumentList_Flutter_InstanceCreationExpression_1() async {
     configureFlutterPkg({
       'src/widgets/framework.dart': flutter_framework_code,
     });
 
     addTestSource('''
 import 'package:flutter/src/widgets/framework.dart';
 
 build() => new Row(
     key: null,
@@ skipping 122 matching lines @@
 ''');
 
     await computeSuggestions(options: generateChildrenBoilerPlate);
 
     assertSuggest('children: ',
         csKind: CompletionSuggestionKind.NAMED_ARGUMENT,
         relevance: DART_RELEVANCE_NAMED_PARAMETER,
         defaultArgListString: null);
   }
 }