flutter: add Linux support

[izzalDev]

Add Linux support to Flutter cask

Summary:
This pull request extends the existing flutter cask to support Linux platforms.
It uses the official Flutter SDK binaries distributed by Google, ensuring parity with the macOS version.

Changes:

• Added corresponding sha256 and url for the official Linux archive.
• Ensured installation, uninstallation, and binaries (flutter, dart) work consistently on both macOS and Linux.
• No behavioral or structural changes for macOS users.

Checklist:

• The submission targets a stable release.
• brew audit --cask --online flutter passes (expected lipo warning due to wrapper scripts).
• brew style --fix flutter reports no offenses.
• brew audit --cask --new flutter passes successfully on Linux.
• HOMEBREW_NO_INSTALL_FROM_API=1 brew install --cask flutter works correctly on both macOS and Linux.
• brew uninstall --cask flutter works correctly on both macOS and Linux.

Notes:

• Flutter’s flutter and dart executables are shell wrappers that invoke platform binaries; lipo warnings during audit are expected and do not indicate an issue.
• Both platforms now install Flutter into $(brew --prefix)/share/flutter for consistent path usage across systems.

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[p-linnane]

Adding an x86_64 dependency on Linux doesn't seem to stop CI from trying other test on arm64 Linux. We might need changes in Homebrew/brew to handle this. Thoughts @Homebrew/cask?

[izzalDev]

The suite stanza does not work on Linux, so I replaced it with artifact.
This change provides similar behavior but works cross-platform, including Linux.

[p-linnane]

Can you elaborate on it 'does not work'?

[bevanjkay]

Just trying something here that may work, ultimately we do need some changes in Homebrew/Brew to handle a couple of cases;

1. Allowing a cask to not supply a checksum for architectures that are not supported.
2. Making sure the brew generate-cask-ci-matrix --cask <token> can correctly identify which architectures to test in these more complicated scenarios, as it will want to test on arm linux even though it isn't supported here.
3. Extending support for suite to Linux casks

Note: I have temporarily changed the suite stanza back to artifact for testing purposes here, but we should look at suite support before we move ahead with this.

[SMillerDev]

Yeah, part of 2 is already part of Homebrew/brew#20334. So you can copy from there.

[izzalDev]

I’m not sure why Flutter doesn’t provide a precache bundle for the Linux ARM64 platform on its official site. However, Flutter does support Linux ARM64 on the stable channel through the stable branch by cloning the official Flutter repository and running flutter precache.

This approach works for all supported platforms and architectures, so I modified this Homebrew cask to automate that process using the source from the stable branch. I’ve tested this cask on Linux x64, Linux ARM64, and macOS ARM64 — all work correctly.

Should this be implemented as a formula instead of a cask? Also, why doesn’t the CI workflow detect support for Linux x64 and Linux ARM64?

[p-linnane]

Why doesn’t the CI workflow detect support for Linux x64 and Linux ARM64?

Linux casks are a relatively new concept, so things like this are still identified as we add them.