Added two more observing changes samples

@sigmundch @jakemac53 PTAL.
an idea to make it slightly more Dart-y: make this line a method in `Thing`, for example:

```dart
class Thing extends Observable { 
   ...
   toggleColor() {
     thing.color = thing.color == 'red' ? 'green' : 'red';
    }
}
```

and do
```dart
   void changeColor(_) => thing.toggleColor();
```

here