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

pkg/front_end/test/dependency_grapher_test.dart

Index: pkg/front_end/test/dependency_grapher_test.dart
diff --git a/pkg/front_end/test/dependency_grapher_test.dart b/pkg/front_end/test/dependency_grapher_test.dart
index 826283d9d81bf8570db8dfd69dbf64c629ef1866..bc1a4729722dbfd4187fc515e5982e7b99fbfc4a 100644
--- a/pkg/front_end/test/dependency_grapher_test.dart
+++ b/pkg/front_end/test/dependency_grapher_test.dart
@@ -35,7 +35,7 @@ class DependencyGrapherTest {
     // 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);
     });