_serializeRest() {
        this.lastChildInserted = true;
        if (!this.defaultSlotsInserted) {
            const defaultSlots = this.slotMap.get('default');
            defaultSlots && defaultSlots.forEach(this._serializeNode);
        }
        this.pushBuffer();
        this.serializedList.push({ placeholder: 'async' });
    }
Serialize the nodes in default slot from slot map and insert async placeholder.

should happen before closing the body.