Allow threads to exit an isolate during rendezvous.

runtime/vm/thread_registry.cc

Index: runtime/vm/thread_registry.cc
diff --git a/runtime/vm/thread_registry.cc b/runtime/vm/thread_registry.cc
index eb11869b1f6eedcfe85cf5d729cd44b8dedc163d..5d23ed69ba06c34da7d7ef79f131b3682c080983 100644
--- a/runtime/vm/thread_registry.cc
+++ b/runtime/vm/thread_registry.cc
@@ -13,6 +13,8 @@ void ThreadRegistry::SafepointThreads() {
   MonitorLocker ml(monitor_);
   // First wait for any older rounds that are still in progress.
   while (in_rendezvous_) {
+    // Assert we are not the organizer trying to nest calls to SafepointThreads.
+    ASSERT(remaining_ > 0);
     CheckSafepointLocked();
   }
   // Start a new round.