Change ObservableComposite to take the same type parameter as ChangeNotificationProvider (it's superclass).
Change ObservableComposite to take the same type parameter as ChangeNotificationProvider (it's superclass).

This allows, e.g. ConstructionLayoutEngine's [_changes] to specify a type parameter, so that the stream listener (`_changes.stream.listen(_checkForDirty)` on line 139) can use types.

This is to avoid a #uses_dynamic_as_bottom error, which will begin failing dart builds soon.

PiperOrigin-RevId: 184788495

angulardart/angular_components,angulardart/angular_components,angulardart/angular_components