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

pkg/front_end/lib/src/incremental/file_state.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:convert';
 import 'dart:typed_data';
 
 import 'package:convert/convert.dart';
 import 'package:crypto/crypto.dart';
 import 'package:front_end/file_system.dart';
 import 'package:front_end/src/base/resolve_relative_uri.dart';
 import 'package:front_end/src/dependency_walker.dart' as graph;
 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/format.dart';
 import 'package:front_end/src/incremental/unlinked_unit.dart';
 import 'package:kernel/target/vm.dart';
 
 /// This function is called for each newly discovered file, and the returned
 /// [Future] is awaited before reading the file content.
 typedef Future<Null> NewFileFn(Uri uri);
 
 /// Information about a file being compiled, explicitly or implicitly.
 ///
@@ skipping 485 matching lines @@
       cycle.libraries.add(node.file);
       fileToCycleMap[node.file] = cycle;
     }
     topologicallySortedCycles.add(cycle);
   }
 
   _LibraryNode getNode(FileState file) {
     return nodesOfFiles.putIfAbsent(file, () => new _LibraryNode(this, file));
   }
 }