Update old examples involving implicit downcasts

[parlough]

Update examples in analyzer documentation and type system documentation around implicit downcasts since they were removed with null safety. There may be other cases as well.

The example in Enabling stricter type checks is no longer relevant as it won't work without the option anyway.

As we migrate The Dart type system article we'll need to keep this in mind as well.

[parlough]

@bwilkerson Does implicit-casts: false have any remaining functionality in >= 2.12 that should be documented or should I just add a note explaining it is as no longer functional?

analyzer:
  strong-mode:
    implicit-casts: false

Sidenote: If it doesn't have any functionality anymore, perhaps there could be a hint/lint with an explanation why it's not useful anymore.

[bwilkerson]

I think that's been replaced by other flags, but @srawlins would know more definitively. But yes, I agree that if it's deprecated / dead then we should remove it from the external docs and have a warning for it.

[srawlins]

implicit-casts still reports when a cast is made from dynamic to another type (if I'm not mistaken), but Dart 2.12 with null safety allows this, so it still has functionality.

[kwalrath]

If you're using 2.12/2.13 but your lower SDK bound is before that (i.e. you haven't opted into null safety), does implicit-casts still catch non-dynamic casts?

(I'm wondering whether we need to have wording that reflects language version rather than SDK version.)

[srawlins]

That's correct, it would.