asset_not_string_or_map

An asset value is required to be a file path (string) or map.

The analyzer produces this diagnostic when an asset value isn't a string or a map.

The following code produces this diagnostic because the asset value is a list:

name: example
flutter:
  assets:
    - [one, two, three]

If you need to specify more than just the path to the asset, then replace the value with a map with a path key (a valid POSIX-style file path):

name: example
flutter:
  assets:
    - path: assets/image.gif
      flavors:
      - premium

If you only need to specify the path, then replace the value with the path to the asset (a valid POSIX-style file path):

name: example
flutter:
  assets:
    - assets/image.gif