dart-event-bus

• Update environment dependency to support up to Dart v3.

Migrate to null safety.

• Add constructor for custom controllers like RxDart Subject (see #21).
• Remove new keyword.

• Cleanup and update dependencies.

• Migrate to Dart 2.
• BREAKING CHANGE: The on method now has a generic type. The type must be passed as a type argument instead of a method parameter. Change myEventBus.on(MyEventType) to myEventBus.on<MyEventType>().
• BREAKING CHANGE: Every EventBus is now hierarchical so that listeners will also receive events of subtypes of the specified type. This is exactly the way that `Hierarchi...