Clean up wording of client usage expectations

pkg/analysis_server/lib/plugin/analysis/analysis_domain.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.
 
 /**
  * Support for client code that interacts with the analysis domain of an
  * analysis server.
  *
  * Plugins can gain access to the request handler that implements the analysis
  * domain in order to extend the functionality of that domain. The class
@@ skipping 39 matching lines @@
 /**
  * A function that is invoked after the analysis domain has been created and is
  * initialized.
  */
 typedef void SetAnalysisDomain(AnalysisDomain domain);
 
 /**
  * An object that gives plugins access to the analysis domain of the analysis
  * server.
  *
- * Clients are not expected to subtype this class.
+ * Clients may not extend, implement or mix-in this class.
  */
 abstract class AnalysisDomain {
   /**
    * Return the stream that is notified when a new value for the given
    * [result] is computed.
    *
    * This method should be used by plugins that need to perform some additional
    * processing after analysis has completed. One example would be a plugin that
    * needed to send a notification to the client because some data was now
    * invalidated.
    */
   Stream<ComputedResult> onResultComputed(ResultDescriptor result);
 
   /**
    * Schedule sending the given [service] notifications for the given [source]
    * in the given [context].
    */
   void scheduleNotification(
       AnalysisContext context, Source source, AnalysisService service);
 }