Use a monotonic clock in the implementation of Timer.

runtime/bin/eventhandler.cc

Index: runtime/bin/eventhandler.cc
diff --git a/runtime/bin/eventhandler.cc b/runtime/bin/eventhandler.cc
index 9e53d5f67ba27efaa9bba2b9be2cb3c7a5c8b253..bb9a7fec383e7a9daad1f4f77b76114481c2ecf6 100644
--- a/runtime/bin/eventhandler.cc
+++ b/runtime/bin/eventhandler.cc
@@ -131,5 +131,12 @@ void FUNCTION_NAME(EventHandler_SendData)(Dart_NativeArguments args) {
   event_handler->SendData(id, dart_port, data);
 }
 
+
+void FUNCTION_NAME(EventHandler_TimerMillisecondClock)(
+    Dart_NativeArguments args) {
+  int64_t now = TimerUtils::GetCurrentMonotonicMillis();
+  Dart_SetReturnValue(args, Dart_NewInteger(now));
+}
+
 }  // namespace bin
 }  // namespace dart