Move byte_store.dart and file_byte_store.dart to their own subdirectory.

pkg/analyzer/test/src/dart/analysis/file_state_test.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:typed_data';
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
 import 'package:analyzer/src/dart/analysis/file_state.dart';
 import 'package:analyzer/src/dart/analysis/top_level_declaration.dart';
 import 'package:analyzer/src/generated/engine.dart'
     show AnalysisOptions, AnalysisOptionsImpl;
 import 'package:analyzer/src/generated/source.dart';
 import 'package:convert/convert.dart';
 import 'package:crypto/crypto.dart';
 import 'package:front_end/src/base/performace_logger.dart';
-import 'package:front_end/src/incremental/byte_store.dart';
+import 'package:front_end/src/byte_store/byte_store.dart';
 import 'package:mockito/mockito.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import '../../context/mock_sdk.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(FileSystemStateTest);
   });
@@ skipping 795 matching lines @@
   String _p(String path) => provider.convertPath(path);
 
   static String _md5(String content) {
     return hex.encode(md5.convert(UTF8.encode(content)).bytes);
   }
 }
 
 class _GeneratedUriResolverMock extends Mock implements UriResolver {}
 
 class _SourceMock extends Mock implements Source {}