TextDocumentMixin constructor Null safety

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

TextDocumentMixin class

1. Constructors
2. TextDocumentMixin
3. Properties
4. hashCode
5. keywords
6. nGrams
7. paragraphs
8. runtimeType
9. sentences
10. sourceText
11. syllableCount
12. terms
13. tokens
14. zones
15. Methods
16. averageSentenceLength
17. averageSyllableCount
18. fleschKincaidGradeLevel
19. fleschReadingEaseScore
20. noSuchMethod
21. toString
22. wordCount
23. Operators
24. operator ==