Add more tests and argument check to Process.sleep

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 + 1 >= milliseconds);
 }
 
 main() {
+  test(0);
   test(1);
   test(10);
   test(100);
+  Expect.throws(() => sleep(new Duration(milliseconds: -1)),
+                (e) => e is ArgumentError);
 }