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

pkg/front_end/test/src/byte_store/byte_store_test.dart

 // Copyright (c) 2016, 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 'package:front_end/src/incremental/byte_store.dart';
+import 'package:front_end/src/byte_store/byte_store.dart';
 import 'package:mockito/mockito.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(MemoryCachingByteStoreTest);
     defineReflectiveTests(NullByteStoreTest);
   });
 }
@@ skipping 96 matching lines @@
     var store = new NullByteStore();
 
     expect(store.get('1'), isNull);
 
     store.put('1', _b(10));
     expect(store.get('1'), isNull);
   }
 }
 
 class _TestByteStore extends Mock implements ByteStore {}