Follow up on null safety codelab

[kwalrath]

#2983 created the null safety codelab. Here's how we should follow up, in rough order of most urgent/easy to less urgent/easy:

• [x] Add /codelabs/null-safety to the list of codelabs in /codelabs (#3112)
• [x] Listof -> ListOf: https://github.com/dart-lang/site-www/pull/2983#discussion_r598166849 (#3112)
• [x] Add link to this bug from the null safety codelab, and perhaps explicitly mention features that aren't covered (yet) (#3112)
• [x] Retire the null safety tour, in favor of the codelab (#3112)
• [x] Redo the late variable example: https://github.com/dart-lang/site-www/pull/3167
• [ ] Add hints & tests to the examples
• [ ] Add coverage of other features, such as required, ?., and ?..