Make index provisional API.

pkg/analysis_server/lib/src/services/search/search_engine_internal.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.src.search.search_engine;
 
 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/correction/source_range.dart';
 import 'package:analysis_server/src/services/index/index.dart';
 import 'package:analysis_server/src/services/index/indexable_element.dart';
 import 'package:analysis_server/src/services/search/search_engine.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * A [SearchEngine] implementation.
  */
@@ skipping 227 matching lines @@
     IndexableElement indexable = new IndexableElement(element);
     add(indexable, relationship, kind);
   }
 
   Future<List<SearchMatch>> merge() {
     return Future.wait(futures).then((List<List<SearchMatch>> matchesList) {
       return matchesList.expand((matches) => matches).toList();
     });
   }
 }