Make index provisional API.

pkg/analysis_server/test/services/index/store/codec_test.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 test.services.src.index.store.codec;
 
-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/index.dart';
 import 'package:analysis_server/src/services/index/indexable_element.dart';
 import 'package:analysis_server/src/services/index/store/codec.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:unittest/unittest.dart';
 
 import '../../../abstract_single_unit.dart';
 import '../../../mocks.dart';
@@ skipping 381 matching lines @@
 class _StringCodecTest {
   StringCodec codec = new StringCodec();
 
   void test_all() {
     int idA = codec.encode('aaa');
     int idB = codec.encode('bbb');
     expect(codec.decode(idA), 'aaa');
     expect(codec.decode(idB), 'bbb');
   }
 }