onLine property Null safety

1. Dart
2. dart:html
3. Navigator
4. onLine property

Navigator class

1. Properties
2. appCodeName
3. appName
4. appVersion
5. budget
6. clipboard
7. connection
8. cookieEnabled
9. credentials
10. dartEnabled
11. deviceMemory
12. doNotTrack
13. geolocation
14. hardwareConcurrency
15. hashCode
16. language
17. languages
18. maxTouchPoints
19. mediaCapabilities
20. mediaDevices
21. mediaSession
22. mimeTypes
23. nfc
24. onLine
25. permissions
26. persistentStorage
27. platform
28. presentation
29. product
30. productSub
31. runtimeType
32. serviceWorker
33. storage
34. temporaryStorage
35. userAgent
36. vendor
37. vendorSub
38. vr
39. webdriver
40. Methods
41. cancelKeyboardLock
42. getBattery
43. getGamepads
44. getInstalledRelatedApps
45. getUserMedia
46. getVRDisplays
47. noSuchMethod
48. registerProtocolHandler
49. requestKeyboardLock
50. requestMediaKeySystemAccess
51. requestMidiAccess
52. sendBeacon
53. share
54. toString
55. Operators
56. operator ==