Migrate to element2 APIs

[davidmorgan]

Let's land element2, hurrah :)

[kevmoo]

analyze still sad 😢

[davidmorgan]

analyze still sad 😢

That was about pubspecs again ... updated to use the now-published source_gen 3.0.0-dev.

Updated with refs for source_gen_test and source_helper, once these are published I can update again and take a look at the failing pubspec-related tests. I guess, when back from vacation :) feel free to take it over if you particularly want to ;) but for builders we don't actually need to publish prerelease versions, just check everything works ready for the full releases.

Thanks :)

[kevmoo]

I will see if I can help here.

[kevmoo]

I need to update source_helper

[kevmoo]

you might want to rebase!

[davidmorgan]

@kevmoo this looks ready now, could you please review? :)

freezed depends on json_serializable, so I think it's probably worth publishing this dev version, even though it will only be for the benefit of a few people :)