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

pkg/front_end/lib/src/byte_store/file_byte_store.dart

 // Copyright (c) 2015, 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:io';
 import 'dart:isolate';
 
-import 'package:front_end/src/incremental/byte_store.dart';
+import 'package:front_end/src/byte_store/byte_store.dart';
 import 'package:path/path.dart';
 
 /**
  * The request that is sent from the main isolate to the clean-up isolate.
  */
 class CacheCleanUpRequest {
   final String cachePath;
   final int maxSizeBytes;
   final SendPort replyTo;
 
@@ skipping 142 matching lines @@
       tempFile.writeAsBytesSync(bytes);
       File file = _getFileForKey(key);
       tempFile.renameSync(file.path);
     } catch (_) {}
   }
 
   File _getFileForKey(String key) {
     return new File(join(_cachePath, key));
   }
 }