TermSimilarityBase constructor Null safety

1. text_analysis
2. implementation
3. TermSimilarityBase
4. TermSimilarityBase constructor

TermSimilarityBase class

1. Constructors
2. TermSimilarityBase
3. Properties
4. characterSimilarity
5. editDistance
6. editSimilarity
7. hashCode
8. jaccardSimilarity
9. lengthDistance
10. lengthSimilarity
11. other
12. runtimeType
13. similarity
14. startsWithSimilarity
15. term
16. Methods
17. compareTo
18. noSuchMethod
19. toJson
20. toString
21. Operators
22. operator ==