Add a sleep function to dart:io

tests/standalone/io/sleep_test.dart

Index: tests/standalone/io/sleep_test.dart
diff --git a/tests/standalone/io/process_set_exit_code_script.dart b/tests/standalone/io/sleep_test.dart
similarity index 50%
copy from tests/standalone/io/process_set_exit_code_script.dart
copy to tests/standalone/io/sleep_test.dart
index 9a34f052a14619eff51b43f7b0e620f0ee8dbf41..be4d266a8629109434ac406812cc297eec609679 100644
--- a/tests/standalone/io/process_set_exit_code_script.dart
+++ b/tests/standalone/io/sleep_test.dart
@@ -4,8 +4,16 @@
 
 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);
 }