[CI, material_ui, cupertino_ui] Gold testing

[Piinks]

Adds Skia Gold integration (flutter_goldens) to the flutter/packages repository (targeting the flutter-packages Gold instance) to support golden file testing across packages such as material_ui and cupertino_ui.

Temporary Rollout Behavior (TODO(Piinks)): While the flutter-packages Gold dashboard (https://flutter-packages-gold.skia.org) is being stood up, FlutterPostSubmitFileComparator and FlutterPreSubmitFileComparator upload images and results to Skia Gold so the backend can ingest initial data, while logging any SkiaExceptions and returning true to avoid blocking CI until the instance frontend is live.

Nearly identical to flutter/flutter instance, adapting to unique aspects of CI between repos and naming conventions/directory structure for packages.

Part of flutter/flutter#172933

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

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³

[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.

[gemini-code-assist]

Code Review

This pull request integrates Skia Gold into the flutter/packages repository's CI and local testing environments. It adds the goldctl dependency to .ci.yaml targets and introduces several golden file comparators along with a SkiaGoldClient to handle authentication, initialization, and image uploads. Feedback on the changes includes fixing an invalid Dart list literal syntax, correcting the path to result-state.json inside the temp directory, adding HTTP status code checks before processing network responses, handling potential missing environment variables, and catching io.IOException instead of specific socket exceptions for more robust error handling.