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 f90abc9aad16cd7fb8d723cd810d33d6d7c289c8..a81a44dfea7c9850aebde4edd92e6e6e8543aa54 100644
--- a/pkg/front_end/lib/physical_file_system.dart
+++ b/pkg/front_end/lib/physical_file_system.dart
@@ -7,8 +7,6 @@ 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
@@ -21,9 +19,6 @@ 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');
@@ -52,4 +47,16 @@ 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();
 }