FileSpan constructor Null safety

1. Flutter
2. source_span
3. FileSpan
4. FileSpan constructor

FileSpan class

1. Constructors
2. FileSpan
3. Properties
4. context
5. end
6. file
7. hashCode
8. length
9. runtimeType
10. sourceUrl
11. start
12. text
13. Methods
14. compareTo
15. expand
16. highlight
17. message
18. noSuchMethod
19. toString
20. union
21. Operators
22. operator ==