WIP - Gold testing

[Piinks]

WIP

The flutter/packages instance of Gold does not have the frontend up yet. I need to validate the backend can receive first.

Still to do here:

• Validation
• Test clean up - still some copy pasta from flutter/flutter version of flutter_goldens
• Finish doc on this

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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Footnotes

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[gemini-code-assist]

Code Review

This pull request introduces a new internal package, flutter_goldens, to enable Skia Gold testing for packages within the flutter/packages repository. It adds the necessary client logic, environment-aware comparators, and test configurations, with two_dimensional_scrollables serving as the initial implementation example. My review has identified some critical and high-severity issues, including a syntax error that will prevent compilation, incorrect Skia Gold configuration, and fragile logic for naming golden files. I have also noted several medium-severity issues related to code duplication, debugging artifacts, and dependency management that should be addressed to improve maintainability. Overall, this is a significant and valuable addition, and my feedback aims to ensure its correctness and robustness.

[Piinks]

Note self for tomorrow: Use the SDK path environment variable, this appears more stable cross CI platforms -flutter +packages.

[auto-submit]

autosubmit label was removed for flutter/packages/10753, because - The status or check suite Linux_web web_dart_unit_test_wasm_shard_2 master has failed. Please fix the issues identified (or deflake) before re-applying this label.

• The status or check suite Linux_web web_dart_unit_test_shard_2 stable has failed. Please fix the issues identified (or deflake) before re-applying this label.
• The status or check suite Linux_web web_dart_unit_test_shard_2 master has failed. Please fix the issues identified (or deflake) before re-applying this label.
• The status or check suite Linux repo_checks has failed. Please fix the issues identified (or deflake) before re-applying this label.