Generate code that supports `implicit-dynamic: false` (list of all assets)

[noinskit]

The new "list of all assets" feature (values on generated classes) can trigger implicit_dynamic_list_literal warnings when implicit-dynamic: false is configured in the analyzer.

It does not happen if a directory contains all files of the same type, but it does for a directory e.g. mixing svg and png files, making values to be of type dynamic (BTW non-nullable Object would be more precise).

ignore_for_file: implicit_dynamic_list_literal might be enough to work this around.

[wasabeef]

@noinskit Thanks, I'll release that this week.