This script will update release metadata in the cloud, and copy the already-built package to the right location and name on cloud storage.
The release metadata will be located in gs://flutter_infra/releases/releases.json, and the published packages will end up in gs://flutter_infra/releases/<channel>/<platform>/flutter_<platform>_<version><archive suffix>, where <channel>, <platform>, <version>, and <archive suffix> are determined by the script.
At the moment, it only supports dev rolls, but (once we know how those will work) should easily support beta rolls as well.
Generate the "version" file from git tags.
Remove the old VERSION file and mentions of versions in pubspec.yaml files.
Replace the old update_versions.dart script with a new roll_dev.dart script.
Update "flutter channel".
Update "flutter upgrade", including making it transition from alpha to dev.
Update "flutter --version" and "flutter doctor".
