Summary:
Introducing the data structure NavigationRouteStack that focused on managing
navigation routes stack.
The goal is to make <Navigatior /> thinner by moving stack management logic into
its own class and make sure it's well-tested.
Teh next step will be cleaning up <Navigatior /> and add `NavigationRouteStack` to
`NavigationContext`.
of the navigator component.
Summary:
Per offline discussion with @evv, we'd like to deprecate the `onDidFocus` and `onWillFocus`
API that makes it really hard for the descendent children of a navigator to observe its focus
change events.
@public
Since for now the descendent children do have access to the navigator via `this.props.navigator`,
this diff makes it easy to observe the focus change event by doing:
```
this.props.navigator.addListener('willfocus', this._onFocus);
```
The goal is to make the event system in navigator more useful and maintainable.
Test Plan:
Test Video: https://www.facebook.com/pxlcld/mrzS
1. jest: ./Libraries/FBReactKit/js/runTests.js NavigationEventEmitter
2. Load UI Explorer: <Navigator />, see console logs that shows the focus change events fires.
