[esnext] prototype runtime implementation for async functions

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 342a26ea9d8a1d34494c371f73eb25348eb86780..030475eb1066cacbcd923ec874940658dd2e85c6 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -6909,6 +6909,9 @@ class SharedFunctionInfo: public HeapObject {
   // Indicates that this function is a generator.
   DECL_BOOLEAN_ACCESSORS(is_generator)
 
+  // Indicates that this function is an async function.
+  DECL_BOOLEAN_ACCESSORS(is_async)
+
   // Indicates that this function is an arrow function.
   DECL_BOOLEAN_ACCESSORS(is_arrow)