How do I update a placeholder image with an async image?

I'm using a manager class to either pull images from a cache or make a network request. I'm using a placeholder image. What's the best way to replace that placeholder image when the proper image is retrieved?

final ItemManager _manager;
final Item _item;
var _itemImage =
  new Image.asset('assets/images/icons/ic_placeholder.png');

@override
Widget build(BuildContext context) {
  _loadImage();
  return new Container(
    child: _itemImage,
  );
}

_loadImage() async {
  var file = await _manager.itemImageForImageUrl(_item.imageUrl);
  _stickerImage = new Image.file(file);
}