Clean up wording of client usage expectations

pkg/analysis_server/lib/plugin/edit/fix/fix_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.fix.fix_dart;
 
 import 'package:analysis_server/plugin/edit/fix/fix_core.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/error.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * A [FixContributor] that can be used to contribute fixes for errors in Dart
  * files.
  *
- * Clients are expected to extend this class when implementing plugins.
+ * Clients may extend this class when implementing plugins.
  */
-abstract class DartFixContributor extends FixContributor {
+abstract class DartFixContributor implements FixContributor {
   @override
   List<Fix> computeFixes(ResourceProvider resourceProvider,
       AnalysisContext context, AnalysisError error) {
     Source source = error.source;
     if (!AnalysisEngine.isDartFileName(source.fullName)) {
       return Fix.EMPTY_LIST;
     }
     List<Source> libraries = context.getLibrariesContaining(source);
     if (libraries.isEmpty) {
       return Fix.EMPTY_LIST;
     }
     CompilationUnit unit =
         context.resolveCompilationUnit2(source, libraries[0]);
     if (unit == null) {
       return Fix.EMPTY_LIST;
     }
     return internalComputeFixes(resourceProvider, unit, error);
   }
 
   /**
    * Return a list of fixes for the given [error]. The error was reported
    * against the given compilation [unit].
    */
   List<Fix> internalComputeFixes(ResourceProvider resourceProvider,
       CompilationUnit unit, AnalysisError error);
 }