Update the plugin API

pkg/analyzer_plugin/lib/src/utilities/assist/assist.dart

 // Copyright (c) 2017, 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.
 
 import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer_plugin/protocol/protocol_generated.dart';
 import 'package:analyzer_plugin/utilities/assist/assist.dart';
 
 /**
  * A concrete implementation of [AssistCollector].
  */
 class AssistCollectorImpl implements AssistCollector {
   /**
    * The list of assists that have been collected.
    */
   final List<PrioritizedSourceChange> assists = <PrioritizedSourceChange>[];
 
   @override
   void addAssist(PrioritizedSourceChange assist) {
     assists.add(assist);
   }
 }
 
 /**
- * A concrete implementation of [AssistRequest].
+ * A concrete implementation of [DartAssistRequest].
  */
-class AssistRequestImpl implements AssistRequest {
+class DartAssistRequestImpl implements DartAssistRequest {
   @override
   final ResourceProvider resourceProvider;
 
   @override
   final int offset;
 
   @override
   final int length;
 
   @override
   final ResolveResult result;
 
   /**
    * Initialize a newly create request with the given data.
    */
-  AssistRequestImpl(
+  DartAssistRequestImpl(
       this.resourceProvider, this.offset, this.length, this.result);
 }