Adapt implicit cast example for type system page

[parlough]

Adds a section about implicit cast runtime errors to the "Type system" page, based off the Invalid casts section of the deprecated sound problems page.

Contributes to #6265

Staged: https://dart-dev--pr6478-feat-type-system-implicit-casts-j5bzqnf7.web.app/language/type-system#implicit-casts

[dart-github-bot]

Visit the preview URL for this PR (updated for commit 32cb124):

https://dart-dev--pr6478-feat-type-system-implicit-casts-j5bzqnf7.web.app

[antfitch]

Thank you for adding that staged link! A dream: we have those created by default when we stage. ^_^

[antfitch]

@eernstg, adding you as our technical reviewer. Can you double-check the staged doc? If you want to triage this to another technical expert, please do so.

Note to Dart engineers: on reviews where any doc changes are added that modify technical content, we generally need to make sure we have at least one technical writer and one additional engineer on those PRs for review.

[eernstg]

LGTM from a programming language perspective. I recommended a couple of simplifications.

[antfitch]

I think this PR looks great. Once @eernstg's changes are added, it'll be ready to launch!

[parlough]

Thanks for the great feedback @eernstg! It does make a lot more sense to keep it focused on a simple downcast example. I applied your suggested changes and updated the section title to be more explicit as you suggested ("Implicit downcasts from dynamic").

[antfitch]

Want to make sure I'm not blocking you. LGTM ^_^