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

pkg/front_end/lib/src/incremental/kernel_driver.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/file_system.dart';
 import 'package:front_end/src/base/api_signature.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/compiler_context.dart';
 import 'package:front_end/src/fasta/dill/dill_library_builder.dart';
 import 'package:front_end/src/fasta/dill/dill_target.dart';
 import 'package:front_end/src/fasta/kernel/kernel_target.dart';
 import 'package:front_end/src/fasta/kernel/utils.dart';
 import 'package:front_end/src/fasta/ticker.dart';
 import 'package:front_end/src/fasta/uri_translator.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:kernel/binary/ast_from_binary.dart';
 import 'package:kernel/core_types.dart';
 import 'package:kernel/kernel.dart' hide Source;
 import 'package:kernel/src/incremental_class_hierarchy.dart';
 import 'package:kernel/type_environment.dart';
 import 'package:meta/meta.dart';
 
 /// This function is invoked for each newly discovered file, and the returned
 /// [Future] is awaited before reading the file content.
@@ skipping 352 matching lines @@
 
   LibraryCycleResult(this.cycle, this.signature, this.kernelLibraries);
 }
 
 @visibleForTesting
 class _TestView {
   /// The list of [LibraryCycle]s compiled for the last delta.
   /// It does not include libraries which were read from the cache.
   final List<LibraryCycle> compiledCycles = [];
 }