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

pkg/front_end/test/incremental_kernel_generator_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/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/incremental_kernel_generator_impl.dart';
 import 'package:kernel/ast.dart';
 import 'package:kernel/text/ast_to_text.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import 'src/incremental/mock_sdk.dart';
 
 main() {
   defineReflectiveSuite(() {
@@ skipping 357 matching lines @@
     throw fail('No library found with URI "$uri"');
   }
 
   String _getLibraryText(Library library) {
     StringBuffer buffer = new StringBuffer();
     new Printer(buffer, syntheticNames: new NameSystem())
         .writeLibraryFile(library);
     return buffer.toString();
   }
 }