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

runtime/bin/eventhandler_android.cc

Index: runtime/bin/eventhandler_android.cc
diff --git a/runtime/bin/eventhandler_android.cc b/runtime/bin/eventhandler_android.cc
index c9c4b0de306b115e63005d15b7fb72eafee50014..195d70d36b5db7792cf23158b8203b2ec7e84f2d 100644
--- a/runtime/bin/eventhandler_android.cc
+++ b/runtime/bin/eventhandler_android.cc
@@ -336,9 +336,14 @@ 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);