[debug] Don't notify listener of exceptions internal to a desugaring.

src/debug/debug.cc

Index: src/debug/debug.cc
diff --git a/src/debug/debug.cc b/src/debug/debug.cc
index de29b83adde0600972ca7ea85a44947043ad8b9e..4556206e5bfa4d4feddaa1b6e558179668036a0e 100644
--- a/src/debug/debug.cc
+++ b/src/debug/debug.cc
@@ -1722,8 +1722,11 @@ MaybeHandle<Object> Debug::PromiseHasUserDefinedRejectHandler(
 
 
 void Debug::OnException(Handle<Object> exception, Handle<Object> promise) {
-  // In our prediction, try-finally is not considered to catch.
   Isolate::CatchType catch_type = isolate_->PredictExceptionCatcher();
+
+  // Don't notify listener of exceptions that are internal to a desugaring.
+  if (catch_type == Isolate::CAUGHT_BY_DESUGARING) return;
+
   bool uncaught = (catch_type == Isolate::NOT_CAUGHT);
   if (promise->IsJSObject()) {
     Handle<JSObject> jspromise = Handle<JSObject>::cast(promise);