Clean up wording of client usage expectations

pkg/analysis_server/lib/plugin/edit/assist/assist_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.assist.assist_dart;
 
 import 'package:analysis_server/plugin/edit/assist/assist_core.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * An [AssistContributor] that can be used to contribute assists for Dart
  * files.
  *
- * Clients are expected to extend this class when implementing plugins.
+ * Clients may extend this class when implementing plugins.
  */
-abstract class DartAssistContributor extends AssistContributor {
+abstract class DartAssistContributor implements AssistContributor {
   @override
   List<Assist> computeAssists(
       AnalysisContext context, Source source, int offset, int length) {
     if (!AnalysisEngine.isDartFileName(source.fullName)) {
       return Assist.EMPTY_LIST;
     }
     List<Source> libraries = context.getLibrariesContaining(source);
     if (libraries.isEmpty) {
       return Assist.EMPTY_LIST;
     }
     CompilationUnit unit =
         context.resolveCompilationUnit2(source, libraries[0]);
     if (unit == null) {
       return Assist.EMPTY_LIST;
     }
     return internalComputeAssists(unit, offset, length);
   }
 
   /**
    * 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> internalComputeAssists(
       CompilationUnit unit, int offset, int length);
 }