Fix for https://github.com/dart-lang/tools/issues/2481

[slashblog]

• This change addresses the following issue I raised earlier. The original issue has more details (including the cause of the issue and steps to reproduce), but briefly adding an item to a list causes the following exception to be thrown: "Failed to produce valid YAML after modification.". This change fixes this issue.

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

Note: The Dart team is trialing Gemini Code Assist. Don't take its comments as final Dart team feedback. Use the suggestions if they're helpful; otherwise, wait for a human reviewer.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gemini-code-assist]

Code Review

This pull request updates the block list appending logic in yaml_edit to trim trailing whitespace when finding the next newline index. The reviewer identified a performance issue where creating a substring on every append operation leads to O(N) time and memory complexity, as well as a potential RangeError crash if the index becomes negative. An optimized O(1) auxiliary space solution was suggested to scan backwards instead.