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

pkg/front_end/test/incremental_resolved_ast_generator_test.dart

Index: pkg/front_end/test/incremental_resolved_ast_generator_test.dart
diff --git a/pkg/front_end/test/incremental_resolved_ast_generator_test.dart b/pkg/front_end/test/incremental_resolved_ast_generator_test.dart
index 40771d8ca022aaccef60a8a3b69d56ffa69ab1d5..5a8682d443d7050a3b62c721ed42829834d7843c 100644
--- a/pkg/front_end/test/incremental_resolved_ast_generator_test.dart
+++ b/pkg/front_end/test/incremental_resolved_ast_generator_test.dart
@@ -12,7 +12,6 @@ import 'package:analyzer/src/dart/sdk/sdk.dart';
 import 'package:front_end/compiler_options.dart';
 import 'package:front_end/incremental_resolved_ast_generator.dart';
 import 'package:front_end/memory_file_system.dart';
-import 'package:path/path.dart' as pathos;
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
@@ -39,7 +38,7 @@ class IncrementalResolvedAstGeneratorTest {
   static final sdkSummaryUri = Uri.parse('special:sdk_summary');
 
   /// Virtual filesystem for testing.
-  final fileSystem = new MemoryFileSystem(pathos.posix, Uri.parse('file:///'));
+  final fileSystem = new MemoryFileSystem(Uri.parse('file:///'));
 
   /// The object under test.
   IncrementalResolvedAstGenerator incrementalResolvedAstGenerator;