Make index provisional API.

pkg/analysis_server/lib/src/services/index/index.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.index;
 
 import 'dart:async';
 
-import 'package:analysis_server/plugin/index/index_core.dart';
+import 'package:analysis_server/src/provisional/index/index_core.dart';
 import 'package:analysis_server/src/services/index/indexable_element.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/engine.dart';
 
 /**
  * A filter for [Element] names.
  */
 typedef bool ElementNameFilter(String name);
 
 /**
@@ skipping 342 matching lines @@
    */
   static RelationshipImpl getRelationship(String identifier) {
     RelationshipImpl relationship = _RELATIONSHIP_MAP[identifier];
     if (relationship == null) {
       relationship = new RelationshipImpl(identifier);
       _RELATIONSHIP_MAP[identifier] = relationship;
     }
     return relationship;
   }
 }