Track the 'awaiter return' call stack use it to detect uncaught exceptions in async functions

runtime/vm/object_service.cc

Index: runtime/vm/object_service.cc
diff --git a/runtime/vm/object_service.cc b/runtime/vm/object_service.cc
index 31c46cdf9ebcf4e14e6e0dedfa6184ee3a0e8771..a28e9fda169edf7eaca63a02296532e34e9c3565 100644
--- a/runtime/vm/object_service.cc
+++ b/runtime/vm/object_service.cc
@@ -869,6 +869,11 @@ void Code::PrintJSONImpl(JSONStream* stream, bool ref) const {
 }
 
 
+void Code::SetAwaitTokenPositions(const Array& await_token_positions) const {
+  StorePointer(&raw_ptr()->await_token_positions_, await_token_positions.raw());
+}
+
+
 void Context::PrintJSONImpl(JSONStream* stream, bool ref) const {
   JSONObject jsobj(stream);
   // TODO(turnidge): Should the user level type for Context be Context