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

pkg/analysis_server/test/src/plugin/plugin_watcher_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 'dart:typed_data';
 
 import 'package:analysis_server/src/plugin/plugin_locator.dart';
 import 'package:analysis_server/src/plugin/plugin_manager.dart';
 import 'package:analysis_server/src/plugin/plugin_watcher.dart';
 import 'package:analyzer/context/context_root.dart';
 import 'package:analyzer/dart/analysis/session.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
 import 'package:analyzer/src/dart/analysis/driver.dart';
 import 'package:analyzer/src/dart/analysis/file_state.dart';
 import 'package:analyzer/src/dart/analysis/session.dart';
 import 'package:analyzer/src/generated/engine.dart' show AnalysisOptionsImpl;
 import 'package:analyzer/src/generated/source.dart';
 import 'package:front_end/src/base/performace_logger.dart';
-import 'package:front_end/src/incremental/byte_store.dart';
+import 'package:front_end/src/byte_store/byte_store.dart';
 import 'package:path/path.dart' as path;
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import '../../mock_sdk.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(PluginWatcherTest);
   });
@@ skipping 124 matching lines @@
     return null;
   }
 
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 
   @override
   void removedContextRoot(ContextRoot contextRoot) {
     removedContextRoots.add(contextRoot);
   }
 }