Rework contribution sorter to use futures

pkg/analysis_server/lib/src/services/completion/dart/contribution_sorter.dart

Index: pkg/analysis_server/lib/src/services/completion/dart/contribution_sorter.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/contribution_sorter.dart b/pkg/analysis_server/lib/src/services/completion/dart/contribution_sorter.dart
similarity index 69%
rename from pkg/analysis_server/lib/src/services/completion/contribution_sorter.dart
rename to pkg/analysis_server/lib/src/services/completion/dart/contribution_sorter.dart
index dbf995b4114f48b8c1b6912ef8033d236295726f..8279be35eb4c9c8c8392e70546b47f019ac0bd06 100644
--- a/pkg/analysis_server/lib/src/services/completion/contribution_sorter.dart
+++ b/pkg/analysis_server/lib/src/services/completion/dart/contribution_sorter.dart
@@ -2,25 +2,25 @@
 // 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 services.completion.sorter;
+library services.completion.dart.sorter;
+
+import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/provisional/completion/completion_core.dart';
 
 /**
- * The abstract class `ContributionSorter` defines the behavior of objects
+ * The abstract class [DartContributionSorter] defines the behavior of objects
  * that are used to adjust the relevance of an existing list of suggestions.
  * This is a long-lived object that should not maintain state between
  * calls to it's [sort] method.
  */
-abstract class ContributionSorter {
+abstract class DartContributionSorter {
   /**
    * After [CompletionSuggestion]s have been computed,
    * this method is called to adjust the relevance of those suggestions.
-   * Return an [AnalysisRequest] if more analysis is needed,
-   * or `null` if suggestion sorting is complete.
-   * This method should execute quickly and not block.
+   * Return a [Future] that completes when the suggestions have been updated.
    */
-  AnalysisRequest sort(
+  Future sort(
       CompletionRequest request, Iterable<CompletionSuggestion> suggestions);
 }