Track async causal stack traces

runtime/vm/thread_registry.h

Index: runtime/vm/thread_registry.h
diff --git a/runtime/vm/thread_registry.h b/runtime/vm/thread_registry.h
index 878479f9a8765fa83b3ca54f57f7515083f28c20..ec49b9032aa9e7ef3b9ded805137ace8b6a6a5a2 100644
--- a/runtime/vm/thread_registry.h
+++ b/runtime/vm/thread_registry.h
@@ -69,6 +69,8 @@ class ThreadRegistry {
   // - Dart_RunLoop()
   // - IsolateSaver in Dart_NewNativePort
   // - Isolate spawn (function/uri) under FLAG_i_like_slow_isolate_spawn
+  // Similarly, tracking async_stack_trace requires that we always reschedule
+  // on the same thread.
   // We probably need a mechanism to return to the specific thread only
   // for these specific cases. We should also determine if the embedder
   // should allow exiting an isolate with live state in zones/handles in