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

pkg/front_end/test/src/incremental/file_state_test.dart

 // Copyright (c) 2017, 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 'package:front_end/memory_file_system.dart';
 import 'package:front_end/src/fasta/uri_translator_impl.dart';
-import 'package:front_end/src/incremental/byte_store.dart';
+import 'package:front_end/src/byte_store/byte_store.dart';
 import 'package:front_end/src/incremental/file_state.dart';
 import 'package:package_config/packages.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import 'mock_sdk.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(FileSystemStateTest);
@@ skipping 489 matching lines @@
   }
 
   Uri _writeFileDirectives(String path,
       {List<String> imports: const [], List<String> exports: const []}) {
     return writeFile(path, '''
 ${imports.map((uri) => 'import "$uri";').join('\n')}
 ${exports.map((uri) => 'export "$uri";').join('\n')}
 ''');
   }
 }