[async-iteration] implement Async-from-Sync Iterator

src/runtime/runtime-internal.cc

Index: src/runtime/runtime-internal.cc
diff --git a/src/runtime/runtime-internal.cc b/src/runtime/runtime-internal.cc
index 2969edfc76c43b13eb5ace315fd0bf9c1c53e2b7..f7864324d6c27ea1674fdaeefa88ba443fa6ba5a 100644
--- a/src/runtime/runtime-internal.cc
+++ b/src/runtime/runtime-internal.cc
@@ -505,10 +505,18 @@ RUNTIME_FUNCTION(Runtime_AllowDynamicFunction) {
 }
 
 RUNTIME_FUNCTION(Runtime_CreateAsyncFromSyncIterator) {
-  // TODO(caitp): split AsyncFromSyncIterator functionality out of
-  //              https://codereview.chromium.org/2622833002
-  UNREACHABLE();
-  return isolate->heap()->undefined_value();
+  HandleScope scope(isolate);
+  DCHECK_EQ(1, args.length());
+
+  CONVERT_ARG_HANDLE_CHECKED(Object, sync_iterator, 0);
+
+  if (!sync_iterator->IsJSReceiver()) {
+    THROW_NEW_ERROR_RETURN_FAILURE(
+        isolate, NewTypeError(MessageTemplate::kSymbolIteratorInvalid));
+  }
+
+  return *isolate->factory()->NewJSAsyncFromSyncIterator(
+      Handle<HeapObject>::cast(sync_iterator));
 }
 
 }  // namespace internal