circular property Null safety

1. smooth_app
2. smooth_gauge
3. SmoothGauge
4. circular property

SmoothGauge class

1. Constructors
2. SmoothGauge
3. Properties
4. backgroundColor
5. circular
6. color
7. hashCode
8. runtimeType
9. size
10. value
11. width
12. Methods
13. build
14. noSuchMethod
15. toString
16. Operators
17. operator ==