Add method on Value::IsAsyncFunction to detect async functions

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 4df8e0a61714608ebdc59d3ecf1b218839a36fae..dd39c2c8a3b696a58bac968b2d21ebb5e9cda3d5 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -3384,6 +3384,12 @@ bool Value::IsRegExp() const {
   return obj->IsJSRegExp();
 }
 
+bool Value::IsAsyncFunction() const {
+  i::Handle<i::Object> obj = Utils::OpenHandle(this);
+  if (!obj->IsJSFunction()) return false;
+  i::Handle<i::JSFunction> func = i::Handle<i::JSFunction>::cast(obj);
+  return func->shared()->is_async();
+}
 
 bool Value::IsGeneratorFunction() const {
   i::Handle<i::Object> obj = Utils::OpenHandle(this);