Clean up wording of client usage expectations

pkg/analysis_server/lib/plugin/edit/utilities/change_builder_dart.dart

 // 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.
 
 library analysis_server.plugin.edit.utilities.change_builder_dart;
 
 import 'package:analysis_server/plugin/edit/utilities/change_builder_core.dart';
 import 'package:analysis_server/src/utilities/change_builder_dart.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/engine.dart';
 
 /**
  * A [ChangeBuilder] used to build changes in Dart files.
  *
- * Clients are not expected to subtype this class.
+ * Clients may not extend, implement or mix-in this class.
  */
 abstract class DartChangeBuilder extends ChangeBuilder {
   /**
    * Initialize a newly created change builder.
    */
   factory DartChangeBuilder(AnalysisContext context) = DartChangeBuilderImpl;
 }
 
 /**
  * An [EditBuilder] used to build edits in Dart files.
  *
- * Clients are not expected to subtype this class.
+ * Clients may not extend, implement or mix-in this class.
  */
 abstract class DartEditBuilder extends EditBuilder {
   /**
    * The group-id used for the name of a declaration.
    */
   static const String NAME_GROUP_ID = 'NAME';
 
   /**
    * The group-id used for the return type of a function, getter or method.
    */
@@ skipping 105 matching lines @@
    */
   bool writeType(DartType type,
       {bool addSupertypeProposals: false,
       String groupName,
       bool required: false});
 }
 
 /**
  * A [FileEditBuilder] used to build edits for Dart files.
  *
- * Clients are not expected to subtype this class.
+ * Clients may not extend, implement or mix-in this class.
  */
 abstract class DartFileEditBuilder extends FileEditBuilder {}