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

pkg/analysis_server/test/services/correction/flutter_util.dart

 // Copyright (c) 2017, 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.correction.flutter_util;
 
 String flutterPkgLibPath = '/packages/flutter/lib';
 
 String get flutter_framework_code => '''
 class Widget {}
 class RenderObjectWidget extends Widget {}
 class StatelessWidget extends Widget {}
+abstract class StatefulWidget extends Widget { }
 class SingleChildRenderObjectWidget extends RenderObjectWidget {}
 class Transform extends SingleChildRenderObjectWidget {}
 class ClipRect extends SingleChildRenderObjectWidget { ClipRect.rect(){} }
 class AspectRatio extends SingleChildRenderObjectWidget {}
 class Container extends StatelessWidget { Container({child: null, width: null, height: null}){}}
 class Center extends StatelessWidget { Center({child: null, key: null}){}}
 class DefaultTextStyle extends StatelessWidget { DefaultTextStyle({child: null}){}}
 class Row extends Widget { Row({List<Widget> children: null, key: null}){}}
 class GestureDetector extends SingleChildRenderObjectWidget { GestureDetector({child: null, onTap: null}){}}
-class Scaffold extends Widget { Scaffold({body: null}){}}
+class AppBar extends StatefulWidget implements PreferredSizeWidget { AppBar(title: null, color: null, key: null) }
+class Scaffold extends Widget { Scaffold({body: null, PreferredSizeWidget appBar: null}){}}
+class PreferredSizeWidget implements Widget {}
 ''';