Ensure that assumed formatting of properties file is correct

[jonahwilliams]

Description

We've seen several user reports of an exception in this method. Currently we assume that the format is exactly key = value, but it is possible this has changed or users files are corrupted. Add a defensive check to ensure we return a reasonable error if we don't find the desired property instead of crashing.

Related Issues

Fixes #29486

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes tests for all changed/updated/fixed behaviors (See Test Coverage).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

• Yes, this is a breaking change (Please read Handling breaking changes).
• No, this is not a breaking change.

[gspencergoog]