Remove FileSystem.lastModified

pkg/front_end/test/memory_file_system_test.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.
 // SharedOptions=--supermixin
 
 library front_end.test.memory_file_system_test;
 
 import 'dart:convert';
 import 'dart:io' as io;
 
@@ skipping 40 matching lines @@
 
   test_exists_exists() async {
     file.writeAsStringSync('x');
     expect(await file.exists(), true);
   }
 
   test_hashCode_samePath() {
     expect(file.hashCode, entityForPath(join(tempPath, 'file.txt')).hashCode);
   }
 
-  test_lastModified_doesNotExist() async {
-    expect(file.lastModified(), _throwsFileSystemException);
-  }
-
-  test_lastModified_increasesOnEachChange() async {
-    file.writeAsStringSync('x');
-    var mod1 = await file.lastModified();
-    file.writeAsStringSync('y');
-    var mod2 = await file.lastModified();
-    expect(mod2.isAfter(mod1), isTrue);
-
-    var file2 = entityForPath(join(tempPath, 'file2.txt'));
-    file2.writeAsStringSync('z');
-    var mod3 = await file2.lastModified();
-    expect(mod3.isAfter(mod2), isTrue);
-  }
-
   test_path() {
     expect(file.uri, context.toUri(path));
   }
 
   test_readAsBytes_badUtf8() async {
     // A file containing invalid UTF-8 can still be read as raw bytes.
     List<int> bytes = [0xc0, 0x40]; // Invalid UTF-8
     file.writeAsBytesSync(bytes);
     expect(await file.readAsBytes(), bytes);
   }
@@ skipping 198 matching lines @@
   String tempPath;
 
   String join(String path1, String path2, [String path3, String path4]) =>
       pathos.windows.join(path1, path2, path3, path4);
 
   void setUp() {
     tempPath = r'c:\test_file_system';
     fileSystem = new MemoryFileSystem(Uri.parse('file:///c:/cwd'));
   }
 }