UPDATE_MESSAGES constant Null safety

1. baguette
2. MessengerKey
3. MessengerKey
4. UPDATE_MESSAGES constant

MessengerKey class

1. Constructors
2. MessengerKey
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toString
9. Operators
10. operator ==
11. Constants
12. TAG
13. UPDATE_CONVERSATIONS
14. UPDATE_MESSAGES