region property Null safety

1. Flutter
2. dart:html
3. MouseEvent
4. region property

MouseEvent class

1. Constructors
2. MouseEvent
3. Properties
4. altKey
5. bubbles
6. button
7. buttons
8. cancelable
9. client
10. composed
11. ctrlKey
12. currentTarget
13. dataTransfer
14. defaultPrevented
15. detail
16. eventPhase
17. fromElement
18. hashCode
19. isTrusted
20. layer
21. matchingTarget
22. metaKey
23. movement
24. offset
25. page
26. path
27. region
28. relatedTarget
29. runtimeType
30. screen
31. shiftKey
32. sourceCapabilities
33. target
34. timeStamp
35. toElement
36. type
37. view
38. Methods
39. composedPath
40. getModifierState
41. noSuchMethod
42. preventDefault
43. stopImmediatePropagation
44. stopPropagation
45. toString
46. Operators
47. operator ==