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

pkg/front_end/test/src/incremental/test_all.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 'package:test_reflective_loader/test_reflective_loader.dart';
 
-import 'byte_store_test.dart' as byte_store;
 import 'file_state_test.dart' as file_state;
 import 'format_test.dart' as format;
 import 'kernel_driver_test.dart' as kernel_driver;
 
 /// Utility for manually running all tests.
 main() {
   defineReflectiveSuite(() {
-    byte_store.main();
     file_state.main();
     format.main();
     kernel_driver.main();
   }, name: 'incremental');
 }