Add missing mutex protection for timeout queue with mutex

runtime/bin/eventhandler.h

Index: runtime/bin/eventhandler.h
diff --git a/runtime/bin/eventhandler.h b/runtime/bin/eventhandler.h
index 30bc7a38db2468f37ea192ae5ec6024d8f2ef58c..6ac1ef41457a7c297bc9e949a68e4c7a5fcb7cdd 100644
--- a/runtime/bin/eventhandler.h
+++ b/runtime/bin/eventhandler.h
@@ -65,10 +65,12 @@ class TimeoutQueue {
   bool HasTimeout() const { return next_timeout_ != NULL; }
 
   int64_t CurrentTimeout() const {
+    ASSERT(next_timeout_ != NULL);
     return next_timeout_->timeout();
   }
 
   Dart_Port CurrentPort() const {
+    ASSERT(next_timeout_ != NULL);
     return next_timeout_->port();
   }