Remove FileSystem.lastModified

pkg/front_end/lib/src/multi_root_file_system.dart

 // Copyright (c) 2017, 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.
 
 /// A file system that implements [CopilerOptions.multiRoots].
 library front_end.src.multi_roots_file_system;
 
 import 'dart:async';
 
 import 'package:front_end/file_system.dart';
@@ skipping 55 matching lines @@
     }
     return multiRootFileSystem.original.entityForUri(uri);
   }
 
   MultiRootFileSystemEntity(this.multiRootFileSystem, this.uri);
 
   @override
   Future<bool> exists() async => (await delegate).exists();
 
   @override
-  Future<DateTime> lastModified() async => (await delegate).lastModified();
-
-  @override
   Future<List<int>> readAsBytes() async => (await delegate).readAsBytes();
 
   @override
   Future<String> readAsString() async => (await delegate).readAsString();
 }
 
 _normalize(Uri uri) =>
     uri.path.endsWith('/') ? uri : uri.replace(path: '${uri.path}/');