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

src/debug/debug.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/debug/debug.h"
 
 #include <memory>
 
 #include "src/api.h"
 #include "src/arguments.h"
@@ skipping 1704 matching lines @@
 
 
 MaybeHandle<Object> Debug::PromiseHasUserDefinedRejectHandler(
     Handle<JSObject> promise) {
   Handle<JSFunction> fun = isolate_->promise_has_user_defined_reject_handler();
   return Execution::Call(isolate_, fun, promise, 0, NULL);
 }
 
 
 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);
     // Mark the promise as already having triggered a message.
     Handle<Symbol> key = isolate_->factory()->promise_debug_marker_symbol();
     JSObject::SetProperty(jspromise, key, key, STRICT).Assert();
     // Check whether the promise reject is considered an uncaught exception.
     Handle<Object> has_reject_handler;
     ASSIGN_RETURN_ON_EXCEPTION_VALUE(
         isolate_, has_reject_handler,
@@ skipping 837 matching lines @@
 }
 
 
 void LockingCommandMessageQueue::Clear() {
   base::LockGuard<base::Mutex> lock_guard(&mutex_);
   queue_.Clear();
 }
 
 }  // namespace internal
 }  // namespace v8