Add file missed in previous CL and comment clean-up

pkg/analysis_server/lib/edit/fix/fix_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.fix.fix_core;
 
 import 'package:analysis_server/src/protocol.dart' show SourceChange;
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/error.dart';
 
 /**
  * A description of a single proposed fix for some problem.
+ *
+ * Clients are not expected to subtype this class.
  */
 class Fix {
   /**
    * An empty list of fixes.
    */
   static const List<Fix> EMPTY_LIST = const <Fix>[];
 
   /**
    * A comparator that can be used to sort fixes by their relevance. The most
    * relevant fixes will be sorted before fixes with a lower relevance.
@@ skipping 19 matching lines @@
   @override
   String toString() {
     return 'Fix(kind=$kind, change=$change)';
   }
 }
 
 /**
  * An object used to produce fixes for a specific error. Fix contributors are
  * long-lived objects and must not retain any state between invocations of
  * [computeFixes].
+ *
+ * Clients are expected to subtype this class when implementing plugins.
  */
 abstract class FixContributor {
   /**
    * Return a list of fixes for the given [error]. The error was reported
    * after it's source was analyzed in the given [context].
    */
   List<Fix> computeFixes(AnalysisContext context, AnalysisError error);
 }
 
 /**
  * A description of a class of fixes. Instances are intended to hold the
  * information that is common across a number of fixes and to be shared by those
  * fixes. For example, if an unnecessary cast is found then one of the suggested
  * fixes will be to remove the cast. If there are multiple unnecessary casts in
  * a single file, then there will be multiple fixes, one per occurance, but they
  * will all share the same kind.
+ *
+ * Clients are not expected to subtype this class.
  */
 class FixKind {
   /**
    * The name of this kind of fix, used for debugging.
    */
   final String name;
 
   /**
    * The relevance of this kind of fix 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 fix.
    */
   final String message;
 
   /**
    * Initialize a newly created kind of fix to have the given [name],
    * [relevance] and [message].
    */
   const FixKind(this.name, this.relevance, this.message);
 
   @override
   String toString() => name;
 }