Add a sleep function to dart:io

tests/standalone/io/sleep_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.
 
 import "dart:io";
 
+test(int milliseconds) {
+  var watch = new Stopwatch();
+  watch.start();
+  sleep(new Duration(milliseconds: milliseconds));
+  watch.stop();
+  Expect.isTrue(watch.elapsedMilliseconds >= milliseconds);
+}
+
 main() {
-  stdout.write("standard out");
-  stderr.write("standard error");
-  exitCode = 25;
+  test(1);
+  test(10);
+  test(100);
 }