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

pkg/front_end/test/incremental_kernel_generator_test.dart

Index: pkg/front_end/test/incremental_kernel_generator_test.dart
diff --git a/pkg/front_end/test/incremental_kernel_generator_test.dart b/pkg/front_end/test/incremental_kernel_generator_test.dart
index ea2313493efb9f168b61755d36e42501327b3653..77e2e416eb715f7ba3ba17b0b345045d51913d11 100644
--- a/pkg/front_end/test/incremental_kernel_generator_test.dart
+++ b/pkg/front_end/test/incremental_kernel_generator_test.dart
@@ -10,7 +10,6 @@ import 'package:front_end/compiler_options.dart';
 import 'package:front_end/incremental_kernel_generator.dart';
 import 'package:front_end/memory_file_system.dart';
 import 'package:kernel/ast.dart';
-import 'package:path/path.dart' as pathos;
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
@@ -37,7 +36,7 @@ class IncrementalKernelGeneratorTest {
   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.
   IncrementalKernelGenerator incrementalKernelGenerator;