Remove Material dependency from transformed_scrollable_test.dart

[gbolahan507]

Replace MaterialApp with TestWidgetsApp and Colors.blue/Colors.red with named constants to remove the Material library dependency.

Part of #177414

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
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• I added new tests to check the change I am making, or this PR is test-exempt.
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Code Review

This pull request removes the Material library dependency from transformed_scrollable_test.dart by replacing MaterialApp with TestWidgetsApp and defining local constants for colors previously imported from the Material library. The change is correct, and I have one suggestion to add documentation to the new color constants to improve code clarity, in line with the repository's style guide.

[navaronbracke]

The tests don't actually require a specific color it seems? Can you use single channel colors, to make the test easier to read? Then the comments can also be omitted, since these no longer apply.

const Color _kBlue = Color(0xFF0000FF);
const Color _kRed = Color(0xFFFFF0000);

[gbolahan507]

Thanks! Updated to use simple single channel colors.

[gbolahan507]

@navaronbracke Friendly ping for review

[justinmc]

LGTM, thanks for helping out here!