message property Null safety

1. diginotescreen
2. message_item
3. MessageItem
4. message property

MessageItem class

1. Constructors
2. MessageItem
3. Properties
4. hashCode
5. height
6. key
7. message
8. runtimeType
9. width
10. Methods
11. build
12. createElement
13. debugDescribeChildren
14. debugFillProperties
15. noSuchMethod
16. toDiagnosticsNode
17. toString
18. toStringDeep
19. toStringShallow
20. toStringShort
21. Operators
22. operator ==