Key statement: "As Flock ships important bug fixes and features, the Flutter team can then choose to add those to Flutter, on their schedule. " I see a lot of negative comments but I think this will be a net positive over time to the Flutter community and to the technology itself. I think it will also give Google a reality check on the needs of the community.
NodeJS went through something similar. It was tightly controlled by a single company (Joyent) who were juggling too many balls, and progress went slowly. Community people did a fork (io.js) which shipped improvements faster, but they did so in a careful non-shitty way. This made lots of people switch to io.js, so that eventually NodeJS merged with io.js (iirc they just renamed the latest io.js "NodeJS").
I don't know of any other instances of community forks of company-controlled OSS that did something similar though. If this is just one guy who made a blog post, like a sibling comment suggests it might be, then it might not pan out this way.