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

pkg/front_end/test/src/incremental/kernel_driver_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/compiler_options.dart';
 import 'package:front_end/memory_file_system.dart';
 import 'package:front_end/src/base/performace_logger.dart';
 import 'package:front_end/src/base/processed_options.dart';
 import 'package:front_end/src/fasta/kernel/utils.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/kernel_driver.dart';
 import 'package:kernel/ast.dart';
 import 'package:kernel/binary/ast_from_binary.dart';
 import 'package:kernel/target/targets.dart';
 import 'package:kernel/text/ast_to_text.dart';
 import 'package:kernel/verifier.dart';
 import 'package:package_config/src/packages_impl.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
@@ skipping 661 matching lines @@
         .writeLibraryFile(library);
     return buffer.toString();
   }
 
   /// Return the [Uri] for the given Posix [path].
   static Uri _folderUri(String path) {
     if (!path.endsWith('/')) path += '/';
     return Uri.parse('file://$path');
   }
 }