Remove FileSystem.lastModified

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';
 
 /// Abstract interface to file system operations.
 ///
@@ skipping 24 matching lines @@
 abstract class FileSystemEntity {
   /// 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;
 
   /// 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
-  /// [FileSystemException].
-  Future<DateTime> lastModified();
-
   /// 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
   /// [FileSystemException].
   Future<List<int>> readAsBytes();
 
   /// Attempts to access this file system entity as a file and read its contents
   /// as a string.
@@ skipping 11 matching lines @@
  */
 class FileSystemException implements Exception {
   final Uri uri;
   final String message;
 
   FileSystemException(this.uri, this.message);
 
   @override
   String toString() => 'FileSystemException(uri=$uri; message=$message)';
 }