Release process

1. Pick a revision (i.e., git hash in this repository) to release. The revision must have been green on the Linux Engine and Mac Engine bots on the waterfall.
2. Land a pull request in flutter.git that updates engine.version to that git hash. As with all pull requests, you should wait for Travis to give you a green light before landing.
3. Watch the waterfall to make sure it doesn't go red.
4. Watch the cocoon dashboard (if you are not a Googler, get one to do this for you) to make sure no tests fail.

If anything fails, immediately roll back the engine release.