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

pkg/front_end/lib/physical_file_system.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 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
 /// I/O.
 ///
 /// Not intended to be implemented or extended by clients.
 class PhysicalFileSystem implements FileSystem {
   static final PhysicalFileSystem instance = new PhysicalFileSystem._();
 
   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');
     }
     return new _PhysicalFileSystemEntity(Uri.base.resolveUri(uri));
   }
 }
 
 /// Concrete implementation of [FileSystemEntity] for use by
 /// [PhysicalFileSystem].
 class _PhysicalFileSystemEntity implements FileSystemEntity {
   @override
   final Uri uri;
 
   _PhysicalFileSystemEntity(this.uri);
 
   @override
   int get hashCode => uri.hashCode;
 
   @override
   bool operator ==(Object other) =>
       other is _PhysicalFileSystemEntity && other.uri == uri;
 
   @override
   Future<List<int>> readAsBytes() => new io.File.fromUri(uri).readAsBytes();
 
   @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();
 }