[assets] Support for licenses need?

[mkustermann]

We expect many dart packages to bundle various kinds of assets (code, images, fonts, other data, ...). Most likely some of those artifacts are based on open source with open source licenses.

Some open source licenses require applications using the artifacts to tell the end user about those uses.

Due to this, flutter walks the transitive package graph, accumulates <foo-pkg>/LICENSEs and combines all of them, bundles them with the final application.

We could assume the LICENSE of a package will also cover all the licenses of the assets produced by hook/build.dart / hook/link.dart and rely on flutter's existing logic. But

• a package may have a different license from some native code it may use in the hooks (could be 3rd party)
• flutter's handling of license accumulation is unconditional: Even if a package isn't actually used, it's license is still included in the final application bundle.
• the standalone dart build probably has to do something similar as flutter build does

=> One may consider adding support for annotating assets with licenses or making licenses an additional asset type (though one would want to then send the license and the asset to the same linker so the linker can include both or none).