Async: Fail assertions after existing assert.async flows are resolved

Fail assertions after existing `assert.async` flows are resolved.

Fixes #668
Closes #669
When you find a style issue that jscs missed, please check if there's an existing ticket on their end, if not, create one. If the same issue shows up multiple times, comment on it once, mention that there's more, ignore the rest. We can still fix them once jscs finds them.