[vector_graphics] Remove unnecessary Material imports

[ricardodalarme]

Removes unnecessary material imports from vector_graphics.

Part of: flutter/flutter#191322

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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Footnotes

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[gemini-code-assist]

Code Review

This pull request updates the imports in packages/vector_graphics/test/caching_test.dart by replacing package:flutter/material.dart with package:flutter/widgets.dart. There are no review comments, and I have no feedback to provide.