Extract computing UnlinkedUnit(s) and cache them in ByteStore.

pkg/front_end/test/src/incremental/file_state_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/memory_file_system.dart';
 import 'package:front_end/src/fasta/translate_uri.dart';
+import 'package:front_end/src/incremental/byte_store.dart';
 import 'package:front_end/src/incremental/file_state.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import 'mock_sdk.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(FileSystemStateTest);
   });
 }
 
 @reflectiveTest
 class FileSystemStateTest {
+  final byteStore = new MemoryByteStore();
   final fileSystem = new MemoryFileSystem(Uri.parse('file:///'));
   final TranslateUri uriTranslator = new TranslateUri({}, {}, {});
   FileSystemState fsState;
 
   Uri _coreUri;
   List<Uri> _newFileUris = <Uri>[];
 
   void setUp() {
     Map<String, Uri> dartLibraries = createSdkFiles(fileSystem);
     uriTranslator.dartLibraries.addAll(dartLibraries);
     _coreUri = Uri.parse('dart:core');
     expect(_coreUri, isNotNull);
-    fsState = new FileSystemState(fileSystem, uriTranslator, <int>[], (uri) {
+    fsState = new FileSystemState(byteStore, fileSystem, uriTranslator, <int>[],
+        (uri) {
       _newFileUris.add(uri);
       return new Future.value();
     });
   }
 
   test_apiSignature() async {
     var path = '/a.dart';
     var uri = writeFile(path, '');
     FileState file = await fsState.getFile(uri);
 
@@ skipping 462 matching lines @@
   Uri _writeFileDirectives(String path,
       {List<String> imports: const [], List<String> exports: const []}) {
     return writeFile(
         path,
         '''
 ${imports.map((uri) => 'import "$uri";').join('\n')}
 ${exports.map((uri) => 'export "$uri";').join('\n')}
 ''');
   }
 }