Add trailing commas after Flutter instance creation params fixes (flutter-intellij/#f978).

pkg/analysis_server/test/services/correction/fix_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.
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/edit/fix/fix_core.dart';
 import 'package:analysis_server/plugin/edit/fix/fix_dart.dart';
 import 'package:analysis_server/protocol/protocol_generated.dart'
     hide AnalysisError;
@@ skipping 415 matching lines @@
         '''
 class A {
   test(int i) {}
   main() {
     test(1);
   }
 }
 ''');
   }
 
+  test_addMissingRequiredArg_cons_flutter_children() async {
+    addPackageSource(
+        'flutter', 'src/widgets/framework.dart', flutter_framework_code);
+
+    _addMetaPackageSource();
+
+    await resolveTestUnit('''
+import 'package:flutter/src/widgets/framework.dart';
+import 'package:meta/meta.dart';
+
+class MyWidget extends Widget {
+  MyWidget({@required List<Widget> children});
+}
+
+build() {
+  return new MyWidget();
+}
+''');
+
+    await assertHasFix(
+        DartFixKind.ADD_MISSING_REQUIRED_ARGUMENT,
+        '''
+import 'package:flutter/src/widgets/framework.dart';
+import 'package:meta/meta.dart';
+
+class MyWidget extends Widget {
+  MyWidget({@required List<Widget> children});
+}
+
+build() {
+  return new MyWidget(children: <Widget>[],);
+}
+''',
+        target: '/test.dart');
+  }
+
   test_addMissingRequiredArg_cons_single() async {
     _addMetaPackageSource();
     addSource(
         '/libA.dart',
         r'''
 library libA;
 import 'package:meta/meta.dart';
 
 class A {
   A({@required int a}) {}
@@ skipping 6430 matching lines @@
 
   @override
   final CompilationUnit unit;
 
   @override
   final AnalysisError error;
 
   _DartFixContextImpl(this.resourceProvider, this.getTopLevelDeclarations,
       this.analysisContext, this.astProvider, this.unit, this.error);
 }