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

pkg/front_end/test/src/base/processed_options_test.dart

Index: pkg/front_end/test/src/base/processed_options_test.dart
diff --git a/pkg/front_end/test/src/base/processed_options_test.dart b/pkg/front_end/test/src/base/processed_options_test.dart
index 94941f1cd5efcd498721f53f61d5fd5e663a5e08..725c895840bae53b9f8eb7f683fb092314d7d08d 100644
--- a/pkg/front_end/test/src/base/processed_options_test.dart
+++ b/pkg/front_end/test/src/base/processed_options_test.dart
@@ -8,7 +8,6 @@ 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';
 
@@ -20,7 +19,7 @@ main() {
 
 @reflectiveTest
 class ProcessedOptionsTest {
-  final fileSystem = new MemoryFileSystem(pathos.posix, Uri.parse('file:///'));
+  final fileSystem = new MemoryFileSystem(Uri.parse('file:///'));
 
   PackageBundleBuilder _mockSdkSummary;