Fix version

Merge pull request #43 from mnordine/nnbd

Migrate to null safety.

Getting ready for v1.1.0

Fix repo url

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 `HierarchicalEventBus` worked. So `HierarchicalEventBus` has been removed. Use the normal `EventBus` instead.

Fix Issue #13: Improve on() stream when no type is specified

Run tests on console

Small readme change

Update to dart libraries 0.9.0

Change default of SimpleEventBus to sync (same as factory in EventBus)