[esnext] prototype runtime implementation for async functions

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index def3c896c599aaa29dfa34dac3681b40ce57e281..35ff8865b9b6e8d4deace308ec9608674c3aaeba 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -1660,7 +1660,7 @@ void Factory::NewJSArrayStorage(Handle<JSArray> array,
 
 Handle<JSGeneratorObject> Factory::NewJSGeneratorObject(
     Handle<JSFunction> function) {
-  DCHECK(function->shared()->is_generator());
+  DCHECK(function->shared()->is_generator() || function->shared()->is_async());
   JSFunction::EnsureHasInitialMap(function);
   Handle<Map> map(function->initial_map());
   DCHECK_EQ(JS_GENERATOR_OBJECT_TYPE, map->instance_type());
@@ -2066,6 +2066,9 @@ Handle<SharedFunctionInfo> Factory::NewSharedFunctionInfo(
     shared->set_instance_class_name(isolate()->heap()->Generator_string());
     shared->DisableOptimization(kGenerator);
   }
+  if (IsAsyncFunction(kind)) {
+    shared->DisableOptimization(kGenerator);
+  }
   return shared;
 }