ImageCache and ImageProvider changes

class MyImageCache implements ImageCache {
  @override
  bool containsKey(Object key) {
    // Check if your custom cache is tracking this key.
  }

  ...
}

class MyImageProvider extends ImageProvider<Object> {
  @override
  ImageStream resolve(ImageConfiguration configuration) {
    // create stream
    // set up error handling
    // interact with ImageCache
    // call obtainKey/load, etc.
  }
  ...
}

class MyImageProvider extends ImageProvider<Object> {
  @override
  ImageStream createStream(ImageConfiguration configuration) {
    // Return stream, or use super.createStream(),
    // which returns a new ImageStream.
  }

  @override
  void resolveStreamForKey(
    ImageConfiguration configuration,
    ImageStream stream,
    Object key,
    ImageErrorListener handleError,
  ) {
    // Interact with the cache, use the key, potentially call `load`,
    // and report any errors back through `handleError`.
  }
  ...
}