Clean up wording of client usage expectations

pkg/analysis_server/tool/spec/codegen_dart_protocol.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 codegen.protocol;
 
 import 'dart:convert';
 
 import 'package:html/dom.dart' as dom;
 
@@ skipping 53 matching lines @@
     'SourceChange': const ['edits', 'linkedEditGroups'],
     'SourceFileEdit': const ['edits'],
     'TypeHierarchyItem': const ['interfaces', 'mixins', 'subclasses'],
   };
 
   /**
    * The disclaimer added to the documentation comment for each of the classes
    * that are generated.
    */
   static const String disclaimer =
-      'Clients are not expected to subtype this class.';
+      'Clients may not extend, implement or mix-in this class.';
 
   /**
    * Visitor used to produce doc comments.
    */
   final ToHtmlVisitor toHtmlVisitor;
 
   /**
    * Types implied by the API.  This includes types explicitly named in the
    * API as well as those implied by the definitions of requests, responses,
    * notifications, etc.
@@ skipping 1155 matching lines @@
 
   @override
   String get asClosure => '($type value) => ${callback('value')}';
 
   @override
   bool get isIdentity => false;
 
   @override
   String asSnippet(String value) => callback(value);
 }