feat(flutter): add a linux_arm64 toolchain

[pawelchcki]

Problem

PLATFORMS registers linux as x86_64-only, so there is no way to build on arm64 Linux. That is increasingly common on CI (AWS CodeBuild arm images, Ampere/Graviton runners) and on developer machines.

The blocker is real but narrower than it looks: Flutter publishes no linux-arm64 SDK tarball, so there is no archive to point a new platform at.

What Flutter does publish

The architecture-specific pieces exist, per engine revision:

flutter/<engine>/dart-sdk-linux-arm64.zip     200
flutter/<engine>/linux-arm64/artifacts.zip    200   # flutter_tester, gen_snapshot, impellerc, icudtl.dat

And only bin/cache holds native code — the framework sources, the flutter_tools kernel snapshot (a JIT snapshot, so portable) and the templates in the x64 archive are architecture-independent.

Approach

linux_arm64 downloads the x64 release archive, deletes the native pieces of bin/cache along with the stamps that mark them current, and lets the existing fetch-time precache refetch them for the host:

• the launcher's update_dart_sdk.sh selects the Dart SDK by uname -m
• flutter_tools resolves engine artifacts through its own host-platform detection

This is the same bootstrap path a plain git clone of the SDK takes on arm64, which is how people run Flutter on arm64 Linux today.

Points worth reviewing

Fetch is host-restricted. Completing the linux_arm64 repository runs the launcher, so it can only be fetched on an arm64 Linux host. Bazel only fetches the host platform's SDK repository, so this is not a practical restriction — but it is enforced with an explicit error rather than a confusing failure later.

Failure is loud, not silent. _verify_rearchitected checks that dart-sdk/bin/dart and linux-arm64/flutter_tester exist after the precache. If a future release stops refetching them as a side effect of precache, it surfaces at fetch time with an actionable message instead of an exec-format error inside a build action.

One latent trap fixed along the way. Making _host_matches_platform architecture-aware would have silently disabled the linux artifact group on arm64 hosts, since _PRECACHE_GROUP_HOSTS values are OS names, not platform names. Those now route through a separate _host_is_os helper.

Existing platforms are untouched. _archive_platform and _platform_arch are identity mappings for everything except linux_arm64, and _PRECACHE_SENTINELS only gained an {arch} placeholder that substitutes to x64 everywhere else.

Testing

Verified on x86_64 that this is a no-op for the existing path: a full flutter build web --release, an 8-shard test suite and flutter analyze --fatal-infos for a real app all pass through the patched ruleset.

The arm64 path is verified against the published artifact URLs (both return 200 for the pinned engine revision, and I confirmed linux-arm64/artifacts.zip contains flutter_tester, gen_snapshot and impellerc) but has not yet been run end to end on arm64 hardware — that is why this is a draft. I am wiring it into arm64 CI now and will report back with a real run before marking it ready.

Would also like your view on whether _ARCHIVE_ALIASES is the shape you want for this, or whether you would rather see an explicit per-platform archive/artifact descriptor.