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`.
As per the recent fix to the `always_specify_types` lint (https://github.com/dart-lang/linter/issues/199), literal maps and lists are now expected to be explicitly typed.
Running that lint on the repo identifies quite a few spots to update. This focuses on `flutter_driver` and `flutter_sprites` (somewhat arbitrarily) but the changes are fairly representative.
Note there are a number of places where I made a quick judgement on how specific to make the types. Feedback on those is welcome. (Especially as we move forward with more.)
