- Add a custom `PlatformPlugin` that spins up a server waiting for test to request a screenshot.
- When a screenshot is requested the plugin talks to Chrome via the debug port, captures a screenshot, and compares it with a golden file
- This PR also adds proper CLI for `dev/test.dart` with an `ArgParser` for future extension into a proper developer tool for the team.
- As a first couple of features, it adds `--debug` option to launch Chrome in debug mode, a `--target` option to choose a single test to run rather than all tests, and `--shard` option to choose a subset of tests to run.
Limitations:
- While screenshot test will run on Cirrus, they are configured to not fail on Cirrus. Need to solve Chrome version skew.
- Stack maps do not work yet (you get stacks, but they are nonsensical)
- When requesting a single test, build_runner builds all tests anyway
- Nothing but desktop Chrome is supported
- Nothing but Linux is supported
- There's no Chrome version pinning; currently assumed stable Chrome channel
