lastEventId property Null safety

1. Flutter
2. dart:html
3. MessageEvent
4. lastEventId property

MessageEvent class

1. Constructors
2. MessageEvent
3. Properties
4. bubbles
5. cancelable
6. composed
7. currentTarget
8. data
9. defaultPrevented
10. eventPhase
11. hashCode
12. isTrusted
13. lastEventId
14. matchingTarget
15. origin
16. path
17. ports
18. runtimeType
19. source
20. suborigin
21. target
22. timeStamp
23. type
24. Methods
25. composedPath
26. noSuchMethod
27. preventDefault
28. stopImmediatePropagation
29. stopPropagation
30. toString
31. Operators
32. operator ==