Remove FileSystem.lastModified

pkg/analyzer/test/src/summary/resynthesize_kernel_test.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.
 
 library analyzer.test.src.summary.resynthesize_kernel_test;
 
 import 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_ast_factory.dart';
@@ skipping 2204 matching lines @@
   final File file;
 
   _FileSystemEntityAdaptor(this.uri, this.file);
 
   @override
   Future<bool> exists() async {
     return file.exists;
   }
 
   @override
-  Future<DateTime> lastModified() async {
-    return new DateTime.fromMicrosecondsSinceEpoch(file.modificationStamp);
-  }
-
-  @override
   Future<List<int>> readAsBytes() async {
     return file.readAsBytesSync();
   }
 
   @override
   Future<String> readAsString() async {
     return file.readAsStringSync();
   }
 }
 
@@ skipping 144 matching lines @@
   }
 
   /**
    * Get the [Source] object for the given [uri].
    */
   Source _getSource(String uri) {
     return _sources.putIfAbsent(
         uri, () => _analysisContext.sourceFactory.forUri(uri));
   }
 }