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

pkg/front_end/test/src/base/processed_options_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:analyzer/src/summary/format.dart';
 import 'package:front_end/compiler_options.dart';
 import 'package:front_end/memory_file_system.dart';
 import 'package:front_end/src/base/processed_options.dart';
-import 'package:path/path.dart' as pathos;
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(ProcessedOptionsTest);
   });
 }
 
 @reflectiveTest
 class ProcessedOptionsTest {
-  final fileSystem = new MemoryFileSystem(pathos.posix, Uri.parse('file:///'));
+  final fileSystem = new MemoryFileSystem(Uri.parse('file:///'));
 
   PackageBundleBuilder _mockSdkSummary;
 
   PackageBundleBuilder get mockSdkSummary => _mockSdkSummary ??=
       new PackageBundleBuilder(apiSignature: 'mock summary signature');
 
   Future<Null> checkMockSummary(CompilerOptions raw) async {
     var processed = new ProcessedOptions(raw);
     var sdkSummary = await processed.getSdkSummary();
     expect(sdkSummary.apiSignature, mockSdkSummary.apiSignature);
@@ skipping 93 matching lines @@
       ..packagesFileUri = new Uri();
     var processed = new ProcessedOptions(raw);
     var uriResolver = await processed.getUriResolver();
     expect(uriResolver.packages, isEmpty);
   }
 
   void writeMockSummaryTo(Uri uri) {
     fileSystem.entityForUri(uri).writeAsBytesSync(mockSdkSummary.toBuffer());
   }
 }