The script is modeled after a similar script in the engine (see engine's
[format.dart](https://github.com/flutter/engine/blob/main/ci/bin/format.dart)).
It identifies the files that have been changed and checks if their
formatting is correct. It also offers an option to correct formatting
(`--fix`) and an option to check the formatting of all files in the
repro (not just changed ones, `--all-files`).
When we are enforcing dart format this script will be called as part of
presubmit.
