StackData<T> constructor Null safety

1. farmershands
2. stack_data
3. StackData<T>
4. StackData constructor

StackData class

1. Constructors
2. StackData
3. Properties
4. hashCode
5. isEmpty
6. runtimeType
7. Methods
8. clear
9. contains
10. noSuchMethod
11. pop
12. push
13. toString
14. Operators
15. operator ==