Migrate source_gen to null-safety

[simolus3]

Migrates source_gen, tests and the example.

Still blocked by

• build_config, build_runner_core and build_runner not being released yet ✔️
• Get ready to publish 2.0.0. dart_style#1000 (and that being published)

Closes #493

[jakemac53]

It would be good to have a version that allows build version 2.0.0 first (as well as 1.x), and then do this migration. That should allow people to migrate a bit more incrementally.

[jakemac53]

Agreed, should I open another PR for that or is someone working on that already?

Nobody is actively working on it right now afaik, either way :). #516 is the issue which nobody has grabbed yet.

[simolus3]

The only remaining dependency override is in example_usage, so this should be ready for review now :)

[kevmoo]

I think we duplicated work - #520

[simolus3]

I'm ok with closing this PR then, but this isn't great. I spent a non-trivial amount of time on this and announced that I was going to work on this in the tracking issue quite some time ago. Very respectfully, I don't see how I duplicated work here.
Whatever, I'm glad we can get this package migrated.

[kevmoo]

I'm ok with closing this PR then, but this isn't great. I spent a non-trivial amount of time on this and announced that I was going to work on this in the tracking issue quite some time ago. Very respectfully, I don't see how I duplicated work here.
Whatever, I'm glad we can get this package migrated.

You're welcome to be upset here. I'm sorry. There are some non-trivial code paths I wanted to double check.

...and honestly I didn't see your PR before I started.

I'd love your eyes on my review, if you're willing.

Regardless, my sincere apologies. Thanks for your help here!