UnmodifiableMapMixin class

1. Constructors
2. UnmodifiableMapMixin
3. Properties
4. first
5. last
6. entries
7. hashCode
8. isEmpty
9. isNotEmpty
10. keys
11. length
12. runtimeType
13. values
14. Methods
15. addAll
16. clear
17. putIfAbsent
18. remove
19. addEntries
20. cast
21. containsKey
22. containsValue
23. forEach
24. map
25. noSuchMethod
26. removeWhere
27. toString
28. update
29. updateAll
30. Operators
31. operator []=
32. operator ==
33. operator []