Rework Timer interface.

tests/standalone/io/secure_session_resume_test.dart

 // Copyright (c) 2012, 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.
 
 // This test tests TLS session resume, by making multiple client connections
 // on the same port to the same server, with a delay of 200 ms between them.
 // The unmodified secure_server_test creates all sessions simultaneously,
 // which means that no handshake completes and caches its keys in the session
 // cache in time for other connections to use it.
 //
@@ skipping 105 matching lines @@
 
   var server = new SecureTestServer();
   int port = server.start();
 
   EndTest = () {
     Expect.equals(CLIENT_NAMES.length, server.numConnections);
     server.stop();
     keepAlive.close();
   };
 
-  int delay = 0;
-  int delay_between_connections = 300;  // Milliseconds.
+  Duration delay = const Duration(milliseconds: 0);
+  Duration delay_between_connections = const Duration(milliseconds: 300);
 
   for (var x in CLIENT_NAMES) {
-    new Timer(delay, (_) {
+    new Timer(delay, () {
       new SecureTestClient(port, x);
     });
     delay += delay_between_connections;
   }
 }