Use black30 for CupertinoTabBar's border

[Sunbreak]

• 0x4D000000 is known as black30
• 0x4D000000 is used by CupertinoNavigationBar
• 0x4C000000 is only used by CupertinoTabBar

Pre-launch Checklist

• [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• [x] I signed the [CLA].
• [ ] I listed at least one issue that this PR fixes in the description above.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or this PR is [test-exempt].
• [x] All existing and new tests are passing.

[MitchellGoodwin]

Hi @Sunbreak. I think this PR's test picked up an issue that's been fixed. Can you rebase on master to rerun the tests?

[Sunbreak]

Hi @Sunbreak. I think this PR's test picked up an issue that's been fixed. Can you rebase on master to rerun the tests?

Rebased. BTW how to rerun without rebasing?

[flutter-dashboard[bot]]

Golden file changes have been found for this pull request. Click here to view and triage (e.g. because this is an intentional change).

If you are still iterating on this change and are not ready to resolve the images on the Flutter Gold dashboard, consider marking this PR as a draft pull request above. You will still be able to view image results on the dashboard, commenting will be silenced, and the check will not try to resolve itself until marked ready for review.

For more guidance, visit Writing a golden file test for package:flutter.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

Changes reported for pull request #119509 at sha 9fc715cde1b5babe49c637d1300be2f7035e611f

[MitchellGoodwin]

@LongCatIsLooong is there a reason why the color was set and left as 0x4C000000 for the tab border?

[LongCatIsLooong]

Looks like this is the commit that first introduced the color constant: 2aaa7f88b3196730295cd0d439f33b1c6d439c00 . I don't think there's a particular reason, the alpha value is probably 0.3, and 0x4C 0x4D are both close to 0.3 (29.8%, 30.196%) and there doesn't seem to be a practical difference between 0x4C and 0x4D?

[MitchellGoodwin]

Got it. Thank you for the link. At the very least, it will be good to have them be the same value, between the navigation bar and the tab bar.