Remove unused dart:async import

[MichaelRFairhurst]

This is unused from Dart 2.1 and on when dart:core exports Future/Stream

[MichaelRFairhurst]

ping @lukasgit

[MichaelRFairhurst]

Ping once more @lukasgit :)

[lukasgit]

I'm sorry for the delay. I lost two very special people in my life. One a couple of months ago, the other two days ago. I'm back and will work on your PR over the weekend. Thanks for your patience and understanding.

[MichaelRFairhurst]

Wow, that sounds horrible Lukas. :(

I will happily and patiently wait, if you do find time over this weekend I'll be super grateful, and if you don't for any reason, I will be 100% understanding. Wish you the best in the grief process, and that's more important than this PR.

[imaNNeo]

Hi. I am pleased to review your code.
I think there is a lot of format change in your PR.
You can make a separate PR to format the code. Let's keep this PR responsible for what you mentioned in the title (Remove unused dart:async import).

It makes PRs clear to follow up. BTW I think we don't have any formatting rule in this repo.