Sentence constructor Null safety

1. text_analysis
2. text_analysis
3. Sentence
4. Sentence constructor

Sentence class

1. Constructors
2. Sentence
3. Properties
4. hashCode
5. runtimeType
6. source
7. tokens
8. Methods
9. noSuchMethod
10. toString
11. Operators
12. operator ==
13. Static methods
14. fromString