Update messages, and fix typing in test.

pkg/compiler/lib/src/diagnostics/generated/shared_messages.dart

 /*
 DON'T EDIT. GENERATED. DON'T EDIT.
 This file has been generated by 'publish.dart' in the dart_messages package.
 
 Messages are maintained in `lib/shared_messages.dart` of that same package.
 After any change to that file, run `bin/publish.dart` to generate a new version
 of the json, dart2js and analyzer representations.
 */
 // 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.
 import '../messages.dart' show MessageTemplate;
 
 enum SharedMessageKind {
   exampleMessage
 }
 
 const Map<SharedMessageKind, MessageTemplate> TEMPLATES = const <SharedMessageKind, MessageTemplate>{ 
   SharedMessageKind.exampleMessage: const MessageTemplate(
     SharedMessageKind.exampleMessage,
     "#use #named #arguments",
+    howToFix: "an explanation on how to fix things",
     examples: const [
       r'''
       Some multiline example;
       That generates the bug.''',
       const {
       'fileA.dart': r'''
         or a map from file to content.
         again multiline''',
       'fileB.dart': r'''
         with possibly multiple files.
         muliline too''',
       },
     ]
   ),  // Generated. Don't edit.
 };