Migrate to null-safety

[fertrig]

We need a couple of dependencies to migrate to null-safety first. These are their issues:

• Migrate to null safety dart-lang/dart_style#978
• Add support for nullsafety dart-lang/source_gen#493

Once both of those issues are resolved, we can migrate the monarch package.

[fertrig]

The dependencies above are migrated. Working on migrating the monarch code to null safety.

[princestha]

Just waiting for this to push so that I can start to utilize this. When are you expecting this to be ready?

Thank you

[fertrig]

Working on it as we speak. I will post an update in a few hours.

[fertrig]

Done! You can find the details here: https://monarchapp.io/blog/version_0.3.6

Let me know how it goes.