Use a monotonic clock in the implementation of Timer.

runtime/bin/eventhandler_macos.cc

Index: runtime/bin/eventhandler_macos.cc
diff --git a/runtime/bin/eventhandler_macos.cc b/runtime/bin/eventhandler_macos.cc
index edd0485cc06c67bb1dfc7520d3a83e1471e92f2c..faf471b2a0f02e1ac4a1f6664a97fd1ad71f591d 100644
--- a/runtime/bin/eventhandler_macos.cc
+++ b/runtime/bin/eventhandler_macos.cc
@@ -394,7 +394,7 @@ int64_t EventHandlerImplementation::GetTimeout() {
     return kInfinityTimeout;
   }
   int64_t millis = timeout_queue_.CurrentTimeout() -
-      TimerUtils::GetCurrentTimeMilliseconds();
+      TimerUtils::GetCurrentMonotonicMillis();
   return (millis < 0) ? 0 : millis;
 }
 
@@ -402,7 +402,7 @@ int64_t EventHandlerImplementation::GetTimeout() {
 void EventHandlerImplementation::HandleTimeout() {
   if (timeout_queue_.HasTimeout()) {
     int64_t millis = timeout_queue_.CurrentTimeout() -
-        TimerUtils::GetCurrentTimeMilliseconds();
+        TimerUtils::GetCurrentMonotonicMillis();
     if (millis <= 0) {
       DartUtils::PostNull(timeout_queue_.CurrentPort());
       timeout_queue_.RemoveCurrent();