PhotoDataSource constructor Null safety

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

PhotoDataSource class

1. Constructors
2. PhotoDataSource
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. getAlbumPhotos
8. getAllPhotos
9. getPhoto
10. noSuchMethod
11. toString
12. Operators
13. operator ==