DistanceUnits constructor Null safety

1. irte_engine
2. settings_units
3. DistanceUnits
4. DistanceUnits constructor

DistanceUnits class

1. Constructors
2. DistanceUnits
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toString
9. Operators
10. operator ==
11. Static methods
12. isMetricUnit
13. Constants
14. distanceUnits
15. imperialUnit
16. metricUnit