[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 4baa4a8f10cdf6f68d0eecf507cdbb034e21ff2f..d5b0c7e3a3ab161fccab726e6c001381eb1ba19c 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -6846,6 +6846,9 @@ 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)