Summary:
Adds a new script that runs Yarn with --frozen-lockfile, and fails if Yarn finds the lockfile is out of sync.
Keeping the lockfile in sync with package.json will ensure our internal open source tests can run offline, as our offline mirror is updated whenever the lockfile changes.
Reverted yarn.lock to https://raw.githubusercontent.com/facebook/react-native/6eeff75849c9b8bf91592c1b7906b4dab8fba518/yarn.lock, then ran `scripts/circleci/validate_yarn_lockfile.sh`, and verified the script failed as expected.
I then ran Yarn without the --frozen-lockfile flag, and reran the validation script. This time, the script finished successfully.
Pull Request resolved: https://github.com/facebook/react-native/pull/21739
Differential Revision: D10365642
Pulled By: hramos
fbshipit-source-id: 9ab7f03919427a86b12901d4c422ef04dd0839f6
5c5af3e1c9