Update the plugin API

pkg/analyzer_plugin/lib/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.dart';
 import 'package:analyzer_plugin/protocol/protocol_generated.dart';
 import 'package:analyzer_plugin/src/utilities/assist/assist.dart';
 import 'package:analyzer_plugin/utilities/generator.dart';
@@ skipping 13 matching lines @@
 /**
  * An object used to produce assists.
  *
  * Clients may implement this class when implementing plugins.
  */
 abstract class AssistContributor {
   /**
    * Contribute assists for the location in the file specified by the given
    * [request] into the given [collector].
    */
-  void computeAssists(AssistRequest request, AssistCollector collector);
+  void computeAssists(
+      covariant AssistRequest request, AssistCollector collector);
 }
 
 /**
  * A generator that will generate an 'edit.getAssists' response.
  *
  * Clients may not extend, implement or mix-in this class.
  */
 class AssistGenerator {
   /**
    * The contributors to be used to generate the assists.
@@ skipping 43 matching lines @@
   /**
    * Return the offset of the selection within the source for which assists are
    * being requested.
    */
   int get offset;
 
   /**
    * Return the resource provider associated with this request.
    */
   ResourceProvider get resourceProvider;
+}
 
+/**
+ * The information about a requested set of assists when computing assists in a
+ * `.dart` file.
+ *
+ * Clients may not extend, implement or mix-in this class.
+ */
+abstract class DartAssistRequest implements AssistRequest {
   /**
    * The analysis result for the file in which the assists are being requested.
    */
   ResolveResult get result;
 }