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

pkg/front_end/test/src/incremental/hot_reload_e2e_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.
 
 /// Integration test that runs the incremental compiler, runs the compiled
 /// program, incrementally rebuild portions of the app, and triggers a hot
 /// reload on the running program.
 library front_end.incremental.hot_reload_e2e_test;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:front_end/compiler_options.dart';
 import 'package:front_end/file_system.dart';
 import 'package:front_end/incremental_kernel_generator.dart';
 import 'package:front_end/memory_file_system.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/testing/hybrid_file_system.dart';
 import 'package:kernel/ast.dart';
 import 'package:kernel/binary/limited_ast_to_binary.dart';
 import 'package:test/test.dart';
 
 import '../../../tool/vm/reload.dart';
 
 main() {
   IncrementalKernelGenerator compiler;
   MemoryFileSystem fs;
@@ skipping 210 matching lines @@
 
 f() => "$line part2";
 ''';
 
 const sourceB = r'''
 get line => "part1";
 ''';
 
 RegExp observatoryPortRegExp =
     new RegExp("Observatory listening on http://127.0.0.1:\([0-9]*\)/");