Remove FileSystem.lastModified

pkg/front_end/lib/src/incremental/file_state.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.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:typed_data';
 
 import 'package:convert/convert.dart';
 import 'package:crypto/crypto.dart';
@@ skipping 418 matching lines @@
   final Uri uri;
 
   final FileState file;
 
   _FileSystemViewEntry(this.uri, this.file);
 
   @override
   Future<bool> exists() async => _shouldNotBeQueried();
 
   @override
-  Future<DateTime> lastModified() async => _shouldNotBeQueried();
-
-  @override
   Future<List<int>> readAsBytes() async {
     if (file == null) {
       throw new FileSystemException(uri, 'File $uri does not exist.');
     }
     return file.content;
   }
 
   @override
   Future<String> readAsString() async => _shouldNotBeQueried();
 
@@ skipping 62 matching lines @@
       cycle.libraries.add(node.file);
       fileToCycleMap[node.file] = cycle;
     }
     topologicallySortedCycles.add(cycle);
   }
 
   _LibraryNode getNode(FileState file) {
     return nodesOfFiles.putIfAbsent(file, () => new _LibraryNode(this, file));
   }
 }