Fix Mac gen_snapshot by calling TimerUtils::InitOnce

runtime/bin/embedded_dart_io.cc

 // Copyright (c) 2015, 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 "bin/embedded_dart_io.h"
 
 #include "bin/eventhandler.h"
+#include "bin/utils.h"
 #include "bin/thread.h"
 
 namespace dart {
 namespace bin {
 
 void BootstrapDartIo() {
   // Bootstrap 'dart:io' event handler.
   Thread::InitOnce();
+  TimerUtils::InitOnce();
   EventHandler::Start();
 }
 
 }  // namespace bin
 }  // namespace dart