[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 790df6ab6fcd283c97bd48046b1a2078b8c4eaf7..d8bb98bff9b85e79ef6303e0d8e38d6844e19937 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -6916,6 +6916,9 @@ 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)