asset_not_string

Assets are required to be file paths (strings).

The analyzer produces this diagnostic when an assets list contains a value that isn't a string.

The following code produces this diagnostic because the assets list contains a map:

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

Change the assets list so that it only contains valid POSIX-style file paths:

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