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

runtime/bin/eventhandler_linux.cc

Index: runtime/bin/eventhandler_linux.cc
diff --git a/runtime/bin/eventhandler_linux.cc b/runtime/bin/eventhandler_linux.cc
index 8b35818ccd680ccf936c8ab4561e0bf569db23ac..ea62bd5b1a4cd47c0521384812f3cf3ae37f6ccc 100644
--- a/runtime/bin/eventhandler_linux.cc
+++ b/runtime/bin/eventhandler_linux.cc
@@ -361,15 +361,14 @@ 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);
       }
       event_mask &= ~(1 << kErrorEvent);
 
       if (event_mask != 0) {
-        intptr_t old_mask = di->Mask();
         Dart_Port port = di->NextNotifyDartPort(event_mask);
         ASSERT(port != 0);
         UpdateEpollInstance(old_mask, di);