Placeholder issue

Placeholder isn't displaying while new image loading.

Inside the```SliverList``` when new item added to it, placeholder not displayed for new item. Old image is displaying until new one is loading

```dart
ImageFade(placeholder: isDarkTheme
? Graphics.placeholderDarkImage
: Graphics.placeholderLightImage,
image: FirebaseImage(_item.getImageUrl(_channel)),
errorBuilder: ((context, obj) => isDarkTheme
? Graphics.placeholderDarkImage
: Graphics.placeholderLightImage),
duration: const Duration(milliseconds: 150),
syncDuration: const Duration(milliseconds: 0),
fit: BoxFit.cover)
```

Start by reproducing the issue with the provided ImageFade configuration inside a SliverList when a new item is added. Inspect how the image and placeholder are updated during loading; done means the new item shows its placeholder instead of retaining the old image until loading finishes.

Written by the indexing model from the issue text.