[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 b1e858207f62fce2d3d14a152b29aa7b9b19ac84..fb97456d3d5700ec162c0faa32c0a97a990f20bb 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -11948,7 +11948,8 @@ 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();
 
@@ -11959,7 +11960,7 @@ 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;