Move completion APIs to a private location

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

 // Copyright (c) 2014, 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 services.completion.sorter;
 
-import 'package:analysis_server/completion/completion_dart.dart';
 import 'package:analysis_server/src/protocol.dart';
+import 'package:analysis_server/src/provisional/completion/completion_dart.dart';
 
 /**
  * The abstract class `ContributionSorter` 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 {
   /**
    * After [CompletionSuggestion]s have been computed,
    * this method is called to adjust the relevance of those suggestions.
    * The compilation unit and completion node
    * in the given completion context may not be resolved.
    * This method should execute quickly and not block.
    */
   void sort(DartCompletionRequest request,
       Iterable<CompletionSuggestion> suggestions);
 }