Add a sleep function to dart:io

runtime/platform/platform_os_macos.cc

Index: runtime/platform/platform_os_macos.cc
diff --git a/runtime/platform/platform_os_macos.cc b/runtime/platform/platform_os_macos.cc
new file mode 100644
index 0000000000000000000000000000000000000000..ef25cc6063395b0b872674d6741d99706b75671e
--- /dev/null
+++ b/runtime/platform/platform_os_macos.cc
@@ -0,0 +1,21 @@
+// 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.
+
+#include "platform/globals.h"
+#if defined(TARGET_OS_MACOS)
+
+#include "platform/platform_os.h"
+
+#include <unistd.h>  // NOLINT
+
+namespace platform {
+
+void OS::Sleep(int64_t millis) {
+  // TODO(5411554):  For now just use usleep we may have to revisit this.
+  usleep(millis * 1000);
+}
+
+}  // namespace platform
+
+#endif  // defined(TARGET_OS_MACOS)