[esnext] prototype runtime implementation for async functions

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 86bdc7cbb268078db55c5e019a891e5a1f1cc659..69fcbb60e09fb18364e1eace8c44d19f7f7fa819 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -12525,7 +12525,8 @@ bool CanSubclassHaveInobjectProperties(InstanceType instance_type) {
 
 
 void JSFunction::EnsureHasInitialMap(Handle<JSFunction> function) {
-  DCHECK(function->IsConstructor() || function->shared()->is_generator());
+  DCHECK(function->IsConstructor() || function->shared()->is_generator() ||
+         function->shared()->is_async());
   if (function->has_initial_map()) return;
   Isolate* isolate = function->GetIsolate();
 
@@ -12536,7 +12537,7 @@ void JSFunction::EnsureHasInitialMap(Handle<JSFunction> function) {
   // First create a new map with the size and number of in-object properties
   // suggested by the function.
   InstanceType instance_type;
-  if (function->shared()->is_generator()) {
+  if (function->shared()->is_generator() || function->shared()->is_async()) {
     instance_type = JS_GENERATOR_OBJECT_TYPE;
   } else {
     instance_type = JS_OBJECT_TYPE;