[google_fonts] Add WOFF and WOFF2 font format support for web

[Frank3K]

Description

Web applications benefit significantly from using WOFF2 font files instead of TTF/OTF. WOFF2 is a compressed format
specifically designed for web delivery, resulting in smaller file sizes and faster load times. This is especially
important for Flutter web apps where every byte counts for initial page load performance.

Size comparison (Roboto Regular):

• TTF: 146,004 bytes
• WOFF2: 63,424 bytes (~56% smaller)

Currently, google_fonts only supports TTF and OTF formats, even on web platforms where WOFF2 would be more efficient.
This PR adds support for WOFF2 and WOFF formats on web, allowing developers to bundle more efficient font files for their
web applications.

What

This PR adds support for .woff2 and .woff font file extensions when running on web platforms, while maintaining
backward compatibility with .ttf and .otf files.

Changes:

• Add kIsWeb check to support web-optimized font formats (.woff2, .woff) in addition to .ttf and .otf
• Make findFamilyWithVariantAssetPath testable by adding an optional isWeb parameter (defaults to kIsWeb)
• Add @visibleForTesting annotation to enable comprehensive testing without exposing internal API
• Add 20 tests covering web-specific, non-web-specific, and common behavior

Platform behavior:

• Web: Supports .woff2, .woff, .ttf, .otf
• Non-web: Supports .ttf, .otf (unchanged)

Implementation Notes

The implementation maintains the existing behavior where the order of assets in the manifest determines which file type
is chosen when multiple formats are available for the same font. While this feels a bit odd (ideally, there would be a
preference order), this is how the function already works for TTF/OTF. In practice, this is not an issue when only one
file type per font is bundled, which is the common case.

Getting WOFF2 Files

Developers can convert TTF/OTF fonts to WOFF2 using Google's woff2_compress tool:
https://github.com/google/woff2

Example:

woff2_compress Roboto-Regular.ttf

Testing

All existing tests pass, plus 20 new tests that verify:

• ✅ WOFF2 and WOFF formats work on web
• ✅ WOFF2 and WOFF formats are ignored on non-web platforms
• ✅ TTF and OTF continue to work on all platforms
• ✅ Common behavior (null handling, family/variant matching) works consistently

Breaking Changes

None. This is a backward-compatible addition that only expands the supported file formats on web platforms.
EOF

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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Footnotes

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[gemini-code-assist]

Code Review

This pull request adds support for WOFF and WOFF2 font formats on the web, which is a great performance improvement due to smaller file sizes. The changes are well-implemented, introducing a check for kIsWeb to conditionally allow the new formats. The findFamilyWithVariantAssetPath function has been thoughtfully refactored to be testable, and the new tests are comprehensive, covering web, non-web, and common scenarios. The decision to maintain the existing asset selection logic based on manifest order to ensure backward compatibility is sensible. Overall, this is a high-quality contribution.