VM eventhandler: Read "old" event mask before it can be modified.

runtime/bin/eventhandler_fuchsia.cc

Index: runtime/bin/eventhandler_fuchsia.cc
diff --git a/runtime/bin/eventhandler_fuchsia.cc b/runtime/bin/eventhandler_fuchsia.cc
index 667e9b05f3cdacb1db2a435e3c93f7e67fabd93c..043e95ee189326a86aad4228da8e354f53ea5246 100644
--- a/runtime/bin/eventhandler_fuchsia.cc
+++ b/runtime/bin/eventhandler_fuchsia.cc
@@ -388,8 +388,8 @@ void EventHandlerImplementation::HandleEvents(struct epoll_event* events,
     } else {
       DescriptorInfo* di =
           reinterpret_cast<DescriptorInfo*>(events[i].data.ptr);
+      const intptr_t old_mask = di->Mask();
       intptr_t event_mask = GetPollEvents(events[i].events, di);
-
       if ((event_mask & (1 << kErrorEvent)) != 0) {
         di->NotifyAllDartPorts(event_mask);
       }
@@ -397,7 +397,6 @@ void EventHandlerImplementation::HandleEvents(struct epoll_event* events,
 
       LOG_INFO("HandleEvents: fd=%ld events=%ld\n", di->fd(), event_mask);
       if (event_mask != 0) {
-        intptr_t old_mask = di->Mask();
         Dart_Port port = di->NextNotifyDartPort(event_mask);
         ASSERT(port != 0);
         UpdateEpollInstance(old_mask, di);