AlbumDataSource constructor Null safety

1. next_photo
2. features
3. AlbumDataSource
4. AlbumDataSource constructor

AlbumDataSource class

1. Constructors
2. AlbumDataSource
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. getAlbum
8. getAllAlbums
9. getUserAlbums
10. noSuchMethod
11. toString
12. Operators
13. operator ==