Remove deprecated <, <=, >, and >= of DateTime.

tests/standalone/io/file_test.dart

 // Copyright (c) 2013, 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.
 //
 // Dart test program for testing file I/O.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
@@ skipping 1052 matching lines @@
           port.toSendPort().send(1);
         });
       });
     });
   }
 
   static void testLastModified() {
     var port = new ReceivePort();
     new File(new Options().executable).lastModified().then((modified) {
       Expect.isTrue(modified is DateTime);
-      Expect.isTrue(modified < new DateTime.now());
+      Expect.isTrue(modified.isBefore(new DateTime.now()));
       port.close();
     });
   }
 
   static void testLastModifiedSync() {
     var modified = new File(new Options().executable).lastModifiedSync();
     Expect.isTrue(modified is DateTime);
-    Expect.isTrue(modified < new DateTime.now());
+    Expect.isTrue(modified.isBefore(new DateTime.now()));
   }
 
   // Test that opens the same file for writing then for appending to test
   // that the file is not truncated when opened for appending.
   static void testAppend() {
     var file = new File('${tempDirectory.path}/out_append');
     file.open(mode: FileMode.WRITE).then((openedFile) {
       openedFile.writeString("asdf").then((ignore) {
         openedFile.close().then((ignore) {
           file.open(mode: FileMode.APPEND).then((openedFile) {
@@ skipping 145 matching lines @@
       testDirectorySync();
       testWriteStringUtf8();
       testWriteStringUtf8Sync();
     });
   }
 }
 
 main() {
   FileTest.testMain();
 }