Add support for nullsafety

[vanlooverenkoen]

I am upgrading my package (Kiwi) gbtb16/kiwi#51

But it seems that source_gen is not yet migrated to support nullsafety. Is there a timeline on when this could be added?

[jakemac53]

This will be blocked on package:build, which is blocked on package:analyzer

[vanlooverenkoen]

Blocked because of: dart-lang/build#2920

[kevmoo]

I believe this has been fixed!

[kevmoo]

@vanlooverenkoen – do you want (1) the code in source_gen to be null safe? Or (2) just to support generating null-safe code?

We have (2) done, not (1).

[vanlooverenkoen]

I want the code in code_gen to be nullsafe

[kevmoo]

We need to get all of the downstream packages migrated, first.

You can still USE source_gen to generate null-safe code.

json_serializable does this now!

[simolus3]

After 2.0 of build is released, we'd still be blocked by dart_style (and build_verify not supporting build 2.x, but that's not relevant now).

Apart from that, I'm happy to migrate this package as well if no-one is working on it yet.