The newest $1 billion unicorn in the Bay Area could be Slack, the business collaboration startup from Flickr co-founder Stewart Butterfield. TechCrunch reported on Thursday that Butterfield is making the rounds for his second big funding of the year for his San Francisco-based company. The Information reported similar information earlier this month. Butterfield has reportedly been pointing to Yammer's $1.2 billion sale to Microsoft in 2012 as a precedent for hitting the $1 billion mark at Slack.…