3523 [shared_preferences] Migrate platform plugins to null-safety (cla: yes, nnbd, p: shared_preferences, platform-linux, platform-web)

3526 [shared_preferences] Migrate main package to null-safety (cla: yes, p: shared_preferences, platform-android, platform-ios, platform-macos, platform-web)

3528 [file_selector_web] Fix typo in pubspec. (cla: yes, p: file_selector, platform-web)

3535 [image_picker] Migrate image_picker_web to null-safety (cla: yes, p: image_picker, platform-web)

3536 [video_player] Update video_player_web to point to new video_player_interface (cla: yes, p: video_player, platform-web)