If you are working on a package with an example/ directory, you usually declare the parent folder as a package dependency. Making changes to the package assets and running flutter build web in the consumer package (example/) won't include the assets in the build. I needed to run flutter clean followed by flutter build web to see the assets included in my build folder.

Steps to reproduce

1. Create a Flutter package
2. Create new Flutter app example/
3. Run flutter build web in example/
4. Add an asset to pubspec.yaml in the parent package:

flutter:
  assets:
    - lib/my_script.js

5. Run flutter build web in example/
6. Observe build/web/assets/ directory doesn't contain the new asset
7. Run flutter clean
8. (edit by @andrewkolos) Run flutter build web again
9. Observe the build/web/assets directory now contains the asset