WIP [esnext] implement async iteration proposal

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index 877b3f7c685750688f890c2c16b0e11a44193bc1..7d28cb348fdff0b21499f1a0450211495c05ac9e 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -1515,7 +1515,9 @@ Handle<JSObject> Factory::NewFunctionPrototype(Handle<JSFunction> function) {
   // can be from a different context.
   Handle<Context> native_context(function->context()->native_context());
   Handle<Map> new_map;
-  if (IsResumableFunction(function->shared()->kind())) {
+  if (IsAsyncGeneratorFunction(function->shared()->kind())) {
+    new_map = handle(native_context->async_generator_object_prototype_map());
+  } else if (IsResumableFunction(function->shared()->kind())) {
     // Generator and async function prototypes can share maps since they
     // don't have "constructor" properties.
     new_map = handle(native_context->generator_object_prototype_map());
@@ -1892,7 +1894,8 @@ Handle<JSGeneratorObject> Factory::NewJSGeneratorObject(
   DCHECK(IsResumableFunction(function->shared()->kind()));
   JSFunction::EnsureHasInitialMap(function);
   Handle<Map> map(function->initial_map());
-  DCHECK_EQ(JS_GENERATOR_OBJECT_TYPE, map->instance_type());
+  DCHECK(JS_GENERATOR_OBJECT_TYPE == map->instance_type() ||
+         JS_ASYNC_GENERATOR_OBJECT_TYPE == map->instance_type());
   CALL_HEAP_FUNCTION(
       isolate(),
       isolate()->heap()->AllocateJSObjectFromMap(*map),
@@ -1957,6 +1960,17 @@ Handle<JSIteratorResult> Factory::NewJSIteratorResult(Handle<Object> value,
   return js_iter_result;
 }
 
+Handle<JSAsyncFromSyncIterator> Factory::NewJSAsyncFromSyncIterator(
+    Handle<HeapObject> sync_iterator) {
+  Handle<Map> map(
+      isolate()->native_context()->initial_async_from_sync_iterator_map());
+  Handle<JSAsyncFromSyncIterator> iterator =
+      Handle<JSAsyncFromSyncIterator>::cast(NewJSObjectFromMap(map));
+
+  iterator->set_sync_iterator(*sync_iterator);
+  return iterator;
+}
+
 Handle<JSMap> Factory::NewJSMap() {
   Handle<Map> map(isolate()->native_context()->js_map_map());
   Handle<JSMap> js_map = Handle<JSMap>::cast(NewJSObjectFromMap(map));