WIP [esnext] implement async iteration proposal

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 23bcd150a801325191373928bf7076cce97ec28c..0db8d8e00feb2b6d4a2f95e4f2980abe78f634b3 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -2761,6 +2761,14 @@ void JSObject::JSObjectShortPrint(StringStream* accumulator) {
       accumulator->Add("<JS Generator>");
       break;
     }
+    case JS_ASYNC_GENERATOR_OBJECT_TYPE: {
+      accumulator->Add("<JS AsyncGenerator>");
+      break;
+    }
+    case JS_ASYNC_FROM_SYNC_ITERATOR_TYPE: {
+      accumulator->Add("<JS AsyncFromSyncIterator>");
+      break;
+    }
     // All other JSObjects are rather similar to each other (JSObject,
     // JSGlobalProxy, JSGlobalObject, JSUndetectable, JSValue).
     default: {
@@ -8645,7 +8653,8 @@ Handle<Map> Map::CopyInitialMap(Handle<Map> map, int instance_size,
   DCHECK(*map == JSFunction::cast(constructor)->initial_map() ||
          *map == *isolate->strict_function_map() ||
          *map == *isolate->generator_function_map() ||
-         *map == *isolate->async_function_map());
+         *map == *isolate->async_function_map() ||
+         *map == *isolate->async_generator_function_map());
 #endif
   // Initial maps must always own their descriptors and it's descriptor array
   // does not contain descriptors that do not belong to the map.
@@ -12638,6 +12647,8 @@ bool CanSubclassHaveInobjectProperties(InstanceType instance_type) {
     case JS_DATE_TYPE:
     case JS_FUNCTION_TYPE:
     case JS_GENERATOR_OBJECT_TYPE:
+    case JS_ASYNC_GENERATOR_OBJECT_TYPE:
+    case JS_ASYNC_FROM_SYNC_ITERATOR_TYPE:
     case JS_MAP_ITERATOR_TYPE:
     case JS_MAP_TYPE:
     case JS_MESSAGE_OBJECT_TYPE:
@@ -12711,7 +12722,9 @@ void JSFunction::EnsureHasInitialMap(Handle<JSFunction> function) {
   // suggested by the function.
   InstanceType instance_type;
   if (IsResumableFunction(function->shared()->kind())) {
-    instance_type = JS_GENERATOR_OBJECT_TYPE;
+    instance_type = IsAsyncGeneratorFunction(function->shared()->kind())
+                        ? JS_ASYNC_GENERATOR_OBJECT_TYPE
+                        : JS_GENERATOR_OBJECT_TYPE;
   } else {
     instance_type = JS_OBJECT_TYPE;
   }