CastMessage constructor Null safety

1. flutter_cast_framework
2. cast
3. CastMessage
4. CastMessage constructor

CastMessage class

1. Constructors
2. CastMessage
3. Properties
4. hashCode
5. message
6. namespace
7. runtimeType
8. Methods
9. encode
10. noSuchMethod
11. toString
12. Operators
13. operator ==
14. Static methods
15. decode