Merging a PR contribution

While creating Gerrit reviews is the preferred approach to contribute to the Dart SDK, contributors sometimes create PRs, anyway.

For administrators: GitHub PRs MUST NOT be merged on GitHub for the SDK repo. Merging a GitHub PR directly breaks mirroring and the repository will be closed until mirroring is restored.

Instead, GitHub PRs are automatically synced to Gerrit if their owner has signed the CLA. They can be submitted like any other patch in Gerrit.

• If the creator of the PR has a Gerrit user account, they will be added as a reviewer.
• The owner of the review is the "Copybara Service" user (subject to change).
• Find all open or merged synced PRs in Gerrit here.
• When the Gerrit CL is submitted the PR will be closed automatically.
• In the future, a link to the Gerrit review will be added to the PR.