I'd also like to see Dart on slack to continue (e.g. add a #general room).
Chat is a great format to build community and learn.
I don't think anyone expects Dart team members to be around and could be regarded as an 'unofficial' thing.
We'd also make sure to mention that people should ask questions on stackoverflow or migrate solved problems to there after the fact.
I can also second what Don said about Slack/IRC, I spent a ton of time on IRC but stopped over the last few years, Slack is just a better experience imho and fulfils a similar use case re community/realtime chat.