The new version has an API change. Also, up our SDK requirements to be a
bit more realistic.

It's safe to remove the unneeded `void`s from setters since the blocking issues in the
`always_declare_return_types` lint have been fixed (https://github.com/dart-lang/linter/).  We can also safely flip the bit on  `avoid_return_types_on_setters`.

And fix a zillion omissions this uncovered.