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

pkg/front_end/lib/physical_file_system.dart

Index: pkg/front_end/lib/physical_file_system.dart
diff --git a/pkg/front_end/lib/physical_file_system.dart b/pkg/front_end/lib/physical_file_system.dart
index a81a44dfea7c9850aebde4edd92e6e6e8543aa54..f90abc9aad16cd7fb8d723cd810d33d6d7c289c8 100644
--- a/pkg/front_end/lib/physical_file_system.dart
+++ b/pkg/front_end/lib/physical_file_system.dart
@@ -7,6 +7,8 @@ library front_end.physical_file_system;
 import 'dart:async';
 import 'dart:io' as io;
 
+import 'package:path/path.dart' as p;
+
 import 'file_system.dart';
 
 /// Concrete implementation of [FileSystem] which performs its operations using
@@ -19,6 +21,9 @@ class PhysicalFileSystem implements FileSystem {
   PhysicalFileSystem._();
 
   @override
+  p.Context get context => p.context;
+
+  @override
   FileSystemEntity entityForUri(Uri uri) {
     if (uri.scheme != 'file' && uri.scheme != '') {
       throw new ArgumentError('File URI expected');
@@ -47,16 +52,4 @@ class _PhysicalFileSystemEntity implements FileSystemEntity {
 
   @override
   Future<String> readAsString() => new io.File.fromUri(uri).readAsString();
-
-  @override
-  Future<bool> exists() async {
-    if (await io.FileSystemEntity.isFile(uri.toFilePath())) {
-      return new io.File.fromUri(uri).exists();
-    } else {
-      return new io.Directory.fromUri(uri).exists();
-    }
-  }
-
-  @override
-  Future<DateTime> lastModified() => new io.File.fromUri(uri).lastModified();
 }