Consistently use implements when one interface inherits from another interface

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 'dart:async';
 
 import 'package:analysis_server/plugin/edit/fix/fix_core.dart';
 import 'package:analysis_server/src/services/correction/fix_internal.dart'
     show DartFixContextImpl;
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * An object used to provide context information for [DartFixContributor]s.
  *
  * Clients may not extend, implement or mix-in this class.
  */
-abstract class DartFixContext extends FixContext {
+abstract class DartFixContext implements FixContext {
   /**
    * The [CompilationUnit] to compute fixes in.
    */
   CompilationUnit get unit;
 }
 
 /**
  * A [FixContributor] that can be used to contribute fixes for errors in Dart
  * files.
  *
@@ skipping 18 matching lines @@
     }
     DartFixContext dartContext = new DartFixContextImpl(context, unit);
     return internalComputeFixes(dartContext);
   }
 
   /**
    * Return a list of fixes for the given [context].
    */
   Future<List<Fix>> internalComputeFixes(DartFixContext context);
 }