Move completion APIs to a private location

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

 // Copyright (c) 2015, 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 analysis_server.src.services.completion.completion_dart;
 
-import 'package:analysis_server/completion/completion_core.dart';
-import 'package:analysis_server/completion/completion_dart.dart';
-import 'package:analysis_server/completion/dart/completion_target.dart';
+import 'package:analysis_server/src/provisional/completion/completion_core.dart';
+import 'package:analysis_server/src/provisional/completion/completion_dart.dart';
+import 'package:analysis_server/src/provisional/completion/dart/completion_target.dart';
 import 'package:analysis_server/src/services/completion/completion_core.dart';
 import 'package:analyzer/src/generated/ast.dart';
 
 /**
  * The information about a requested list of completions within a Dart file.
  */
 class DartCompletionRequestImpl extends CompletionRequestImpl
     implements DartCompletionRequest {
   /**
    * The compilation unit in which the completion was requested.
@@ skipping 12 matching lines @@
   final CompletionTarget target;
 
   /**
    * Initialize a newly created completion request based on the given arguments.
    */
   DartCompletionRequestImpl(
       CompletionRequest request, this.unit, this.isResolved, this.target)
       : super(request.context, request.resourceProvider, request.source,
             request.offset);
 }