Description

This PR fixes ThemeData.dividerColor to return the correct color that matches what the Divider widget actually renders in Material 2 and Material 3.

Problem

ThemeData.dividerColor returned different values depending on how the theme was created, causing inconsistency with the actual Divider widget color:

• ThemeData() with M3 returned outline instead of outlineVariant
• ThemeData.from() with M3 returned M2 value onSurface.withOpacity(0.12) instead of M3 value outlineVariant
• Divider widget always correctly used outlineVariant in M3

This created confusion for developers who expected Theme.of(context).dividerColor to match the actual divider color they saw on screen.

Solution

Updated both ThemeData() and ThemeData.from() constructors to use the correct divider colors per Material Design specifications:

• Material 2: ColorScheme.onSurface.withOpacity(0.12) (unchanged)
• Material 3: ColorScheme.outlineVariant (fixed)

Now all theme creation methods return the correct dividerColor that matches what the Divider widget actually renders.

Testing

Added 4 comprehensive tests that verify:

• M3: dividerColor equals outlineVariant for all theme creation methods
• M3: dividerColor matches actual Divider widget color
• M2: dividerColor equals onSurface.withOpacity(0.12) and matches actual Divider widget color

Related Issues

Fixes #117755

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• 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 ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.