Add file missed in previous CL and comment clean-up

pkg/analysis_server/lib/edit/assist/assist_core.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.edit.assist.assist_core;
 
 import 'package:analysis_server/src/protocol.dart' show SourceChange;
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * A description of a single proposed assist.
+ *
+ * Clients are not expected to subtype this class.
  */
 class Assist {
   /**
    * An empty list of assists.
    */
   static const List<Assist> EMPTY_LIST = const <Assist>[];
 
   /**
    * A comparator that can be used to sort assists by their relevance. The most
    * relevant assists will be sorted before assists with a lower relevance.
@@ skipping 18 matching lines @@
   Assist(this.kind, this.change);
 
   @override
   String toString() {
     return 'Assist(kind=$kind, change=$change)';
   }
 }
 
 /**
  * An object used to produce assists for a specific location.
+ *
+ * Clients are expected to subtype this class when implementing plugins.
  */
 abstract class AssistContributor {
   /**
    * Return a list of assists for a location in the given [source]. The location
    * is specified by the [offset] and [length] of the selected region. The
    * [context] can be used to get additional information that is useful for
    * computing assists.
    */
   List<Assist> computeAssists(
       AnalysisContext context, Source source, int offset, int length);
 }
 
 /**
  * A description of a class of assists. Instances are intended to hold the
  * information that is common across a number of assists and to be shared by
  * those assists.
+ *
+ * Clients are not expected to subtype this class.
  */
 class AssistKind {
   /**
    * The name of this kind of assist, used for debugging.
    */
   final String name;
 
   /**
    * The relevance of this kind of assist for the kind of error being addressed.
    */
   final int relevance;
 
   /**
    * A human-readable description of the changes that will be applied by this
    * kind of assist.
    */
   final String message;
 
   /**
    * Initialize a newly created kind of assist to have the given [name],
    * [relevance] and [message].
    */
   const AssistKind(this.name, this.relevance, this.message);
 
   @override
   String toString() => name;
 }