Update the plugin API

pkg/analyzer_plugin/lib/utilities/completion/completion_core.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 'dart:async';
 
 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_common.dart';
@@ skipping 55 matching lines @@
  * An object used to produce completion suggestions.
  *
  * Clients may implement this class when implementing plugins.
  */
 abstract class CompletionContributor {
   /**
    * Contribute completion suggestions for the completion location specified by
    * the given [request] into the given [collector].
    */
   Future<Null> computeSuggestions(
-      CompletionRequest request, CompletionCollector collector);
+      covariant CompletionRequest request, CompletionCollector collector);
 }
 
 /**
  * A generator that will generate a 'completion.getSuggestions' response.
  *
  * Clients may not extend, implement or mix-in this class.
  */
 class CompletionGenerator {
   /**
    * The contributors to be used to generate the completion suggestions.
@@ skipping 48 matching lines @@
    * requested.
    */
   int get offset;
 
   /**
    * Return the resource provider associated with this request.
    */
   ResourceProvider get resourceProvider;
 
   /**
-   * The analysis result for the file in which the completion is being
-   * requested.
-   */
-  ResolveResult get result;
-
-  /**
    * Throw an [AbortCompletion] if the completion request has been aborted.
    */
   void checkAborted();
 }
+
+/**
+ * The information about a requested list of completions when completing in a
+ * `.dart` file.
+ *
+ * Clients may not extend, implement or mix-in this class.
+ */
+abstract class DartCompletionRequest implements CompletionRequest {
+  /**
+   * The analysis result for the file in which the completion is being
+   * requested.
+   */
+  ResolveResult get result;
+}