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

pkg/front_end/lib/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.file_system;
 
 import 'dart:async';
 
-import 'package:path/path.dart' as path;
-
 /// Abstract interface to file system operations.
 ///
 /// All front end interaction with the file system goes through this interface;
 /// this makes it possible for clients to use the front end in a way that
 /// doesn't require file system access (e.g. to run unit tests, or to run
 /// inside a browser).
 ///
 /// Not intended to be implemented or extended by clients.
 abstract class FileSystem {
-  /// Returns a path context suitable for use with this [FileSystem].
-  ///
-  /// TODO(paulberry): try to eliminate all usages of this.  Since the
-  /// FileSystem API now uses URIs rather than paths, it should not be needed.
-  path.Context get context;
-
   /// Returns a [FileSystemEntity] corresponding to the given [uri].
   ///
   /// Uses of `..` and `.` in the URI are normalized before returning.
   ///
   /// If the URI scheme is not supported by this file system, an [Error] will be
   /// thrown.
   ///
   /// Does not check whether a file or folder exists at the given location.
   FileSystemEntity entityForUri(Uri uri);
 }
 
 /// Abstract representation of a file system entity that may or may not exist.
 ///
 /// Instances of this class have suitable implementations of equality tests and
 /// hashCode.
 ///
 /// Not intended to be implemented or extended by clients.
 abstract class FileSystemEntity {
-  /// Returns the absolute normalized URI represented by this file system
-  /// entity.
+  /// The absolute normalized URI represented by this file system entity.
   ///
   /// Note: this is not necessarily the same as the URI that was passed to
   /// [FileSystem.entityForUri], since the URI might have been normalized.
   Uri get uri;
 
   /// Attempts to access this file system entity as a file and read its contents
   /// as raw bytes.
   ///
   /// If an error occurs while attempting to read the file (e.g. because no such
   /// file exists, or the entity is a directory), the future is completed with
   /// an [Exception].
   Future<List<int>> readAsBytes();
 
   /// Attempts to access this file system entity as a file and read its contents
   /// as a string.
   ///
   /// The file is assumed to be UTF-8 encoded.
   ///
   /// If an error occurs while attempting to read the file (e.g. because no such
   /// file exists, the entity is a directory, or the file is not valid UTF-8),
   /// the future is completed with an [Exception].
   Future<String> readAsString();
+
+  /// Whether this file system entity exists.
+  Future<bool> exists();
+
+  /// Extracts the last-modification time of the file system entity, if it
+  /// exists and it is a file, otherwise the future is completed with an
+  /// [Exception].
+  Future<DateTime> lastModified();
 }