[lint] always_declare_return_types

[evanweible-wf]

http://dart-lang.github.io/linter/lints/always_declare_return_types.html

• Overrides have to re-declare the return type, which is particularly annoying for libraries or patterns where this is common like over_react's render() method.

[regenvanwalbeek-wf]

We have upgraded this to error in DPC. I think workiva_analysis_options should consider upgrading this to warning or error in the future.

I haven't seen a much inconvenience in having to re-declare overridden return types. The benefits of preventing methods without declared return types outweigh the costs.