Add async related rules

[devjeff]

I wan to suggest to add the following rules to the effective dart analysis options:

• avoid_void_async
• await_only_futures

I find these lint rules extremely important as the helped me to find and avoid multiple issues in cases where you simply forget to add the async keyword to a method definition and try to await the result, although the method isn't really executed asynchronously. I can not think of a case where it would be reasonable to omit these rules.

[tenhobi]

Hello @devjeff ✋,
this package only reflect official Effective Dart rules, so I cannot simply add a new rule. You can open an issue probably in https://github.com/dart-lang/linter

Thanks anyway!

[devjeff]

Ok. I see. Thanks for your hint.