Extend file-system abstraction with a couple methods, remove context

pkg/front_end/test/dependency_grapher_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 'dart:async';
 
 import 'package:front_end/compiler_options.dart';
 import 'package:front_end/dependency_grapher.dart';
 import 'package:front_end/memory_file_system.dart';
 import 'package:path/path.dart' as pathos;
@@ skipping 17 matching lines @@
         unorderedEquals(dependencies));
     expect(library.parts.map((part) => '$part'), unorderedEquals(parts));
     return library;
   }
 
   Future<List<LibraryCycleNode>> getCycles(Map<String, String> contents,
       {List<String> startingPoints, String packagesFilePath = ''}) async {
     // If no starting points given, assume the first entry in [contents] is the
     // single starting point.
     startingPoints ??= [contents.keys.first];
-    var fileSystem = new MemoryFileSystem(pathos.posix, Uri.parse('file:///'));
+    var fileSystem = new MemoryFileSystem(Uri.parse('file:///'));
     contents.forEach((path, text) {
       fileSystem.entityForUri(pathos.posix.toUri(path)).writeAsStringSync(text);
     });
     // TODO(paulberry): implement and test other option possibilities.
     var options = new CompilerOptions()
       ..fileSystem = fileSystem
       ..chaseDependencies = true
       ..packagesFileUri = packagesFilePath == ''
           ? new Uri()
           : pathos.posix.toUri(packagesFilePath);
@@ skipping 137 matching lines @@
     expect(bar.dependencies[0], same(foo));
   }
 
   test_singleFile() async {
     var cycles = await getCycles({'/foo.dart': ''});
     expect(cycles, hasLength(1));
     expect(cycles[0].libraries, hasLength(1));
     checkLibrary(cycles[0], 'file:///foo.dart');
   }
 }