Remove FileSystem.lastModified

pkg/front_end/lib/src/testing/hybrid_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 memory + physical file system used to mock input for tests but provide
 /// sdk sources from disk.
 library front_end.src.hybrid_file_system;
 
 import 'dart:async';
 
@@ skipping 27 matching lines @@
     return _delegate ??= (await memory.exists()) ? memory : physical;
   }
 
   @override
   Uri get uri => memory.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();
 }