Add missing margin to SnackBarAction

[Albert221]

Description

Fixes #48042. Adds a horizontal margin to the SnackBar action, according to Material Design guidelines.

As you can see on the first screenshot below (before), the SnackBarAction does not have any space between the right button border and the right snackbar border which is not a correct appearance according to Material Design guidelines

Before

After

Related Issues

#48042

Tests

I added the following tests:

• SnackBar has correct end padding when it contains an action with fixed behavior
• SnackBar has correct end padding when it contains an action with floating behavior

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.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read Handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change.
  • I wrote a design doc: https://flutter.dev/go/template
  • I got input from the developer relations team, specifically from:
  • I wrote a migration guide: https://flutter.dev/go/breaking-changes-template

Yes, I changed some of the existing tests, but they weren't following the spec in Material Design guidelines too. That change is really cosmetic, I don't think there is a need for a migration guide or a design doc.

[Piinks]

Hi @Albert221 welcome! And thank you for the contribution. It looks like there is a merge conflict, can you update your change to resolve it?

[Piinks]

It looks like the latest update included the NNBD migration of the test files you are working in, so your tests will need to be null safe. That should resolve the test failures here.

[Albert221]

Of course, here's the fixed version!

[Piinks]

[fluttergithubbot]

This pull request is not suitable for automatic merging in its current state.

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