StreamController are not closed.

While digging through the code I have noticed that the `StreamController` are not closed.  

Ie date_input.dart:  
```dart
final _controller = StreamController<Date?>.broadcast();
``` 

Shouldn't we close it within `ngOnDestroy` to not have a memory leak?  

I guess it would be better if we attach it into `Disposer` and let it clear the stuff.