[tool] Ensure that all packages format with correct language version

[stuartmorgan-g]

This fixes two issues with the format command's language resolution handling:

• It was not running pub get in sub-packages, so for the rare case of sub-packages with different min versions than their enclosing package, format behavior was inconsistent, and incorrect in CI where the tree starts clean.
• It was not detecting stale resolution, which was annoying when formatting locally since it's not uncommon to have last run pub get for some package before the last min SDK bump for that package, causing local formatting to be incorrect. This was especially problematic when doing min SDK bumps for the entire repo after stable releases.

It also fixes the related problem that fetch-deps was not running pub get in sub-packages, which would have prevented the pigeon issue in CI, but not locally. Even with format fixed, we still want everything fetched in fetch-deps.

The parsing does depend on internal implementation details of Dart, but we don't expect those to change frequently, and if it does we can update the detection logic as necessary. The failure mode should either be to throw, which we'll catch during the roll, or to run pub get more than necessary, which would just be an optimization issue rather than a correctness issue.

Includes pigeon format changes, since now CI is enforcing the correct format instead of the incorrect format that was checked in.

Fixes flutter/flutter#178072
Helps with flutter/flutter#176853

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.

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

[stuartmorgan-g]

CHANGELOG/version override: the format change to a small part of example/lib/main.dart isn't worth publishing for; we can just pick it up during the next normal publish.

[stuartmorgan-g]

This is just drive-by refactoring; this helper being here predates the creation of RepositoryPackage, and that class is a better home for this method (especially since it already has a similar method to find all example apps).

It changed its return from Stream to Iterable to match the existing helper function, and because we don't actually benefit from using Stream.

[gemini-code-assist]

Code Review

This pull request improves the format and fetch-deps commands to correctly handle language versions in packages and their sub-packages. The changes ensure that pub get is run when needed, fixing inconsistencies in formatting, especially in CI environments. The logic for detecting stale language version resolution has been added, which is a great improvement for local development workflows. The related refactoring to move getSubpackages into RepositoryPackage and make it synchronous simplifies the code. The included tests for the new logic are comprehensive. I have one suggestion to make the SDK version constraint handling more robust.