[native_toolchain_c] Add linking for macOS

[goderbauer]

Towards #1376

This enables linking for MacOS targets on MacOS hosts.

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
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pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/native_doc_dartifier/example/native_doc_dartifier_example.dart
pkgs/native_doc_dartifier/lib/native_doc_dartifier.dart
pkgs/native_doc_dartifier/lib/src/native_doc_dartifier_base.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:code_assets	0.19.4	already published at pub.dev
package:data_assets	0.19.1	already published at pub.dev
package:ffi	2.1.4	already published at pub.dev
package:ffigen	19.1.0-wip	WIP (no publish necessary)
package:hooks	0.19.5	already published at pub.dev
package:hooks_runner	0.21.0	already published at pub.dev
package:jni	0.14.2	already published at pub.dev
package:jnigen	0.15.0-wip	WIP (no publish necessary)
package:native_doc_dartifier	0.0.1-pre	already published at pub.dev
package:native_toolchain_c	0.16.4	ready to publish	native_toolchain_c-v0.16.4
package:objective_c	8.1.0-wip	WIP (no publish necessary)
package:swift2objc	0.0.1-wip	WIP (no publish necessary)
package:swiftgen	0.0.1-wip	WIP (no publish necessary)

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[goderbauer]

The breaking check still has problems. I verified locally that this isn't a breaking change.

[dcharkes]

The breaking check still has problems. I verified locally that this isn't a breaking change.

I've filed #2361.

[coveralls]

coverage: 88.027% (+0.03%) from 88.001%
when pulling 4a8137d on goderbauer:macos-linker
into 5eadfaf on dart-lang:main.

[dcharkes]

Thanks @goderbauer !

[dcharkes]

Maybe this should be written as a switch on target OS (which then has both the Android and Linux cases got to the same block). Early return for MacOS and combining all other OSes seems somewhat weird.

[dcharkes]

Should we consider throwing argument errors instead of doing asserts?

Especially because below we check macOSTargetVersion != null instead of targetOS == OS.macOS.

[dcharkes]

(Hm, they should have made a bunch of const objects and an Or([Linux(), MacOS()]) class instead of going for strings. How do we know if this syntax is correct and we're not just skipping the test everywhere because of wrong syntax. 😄 )

[goderbauer]

I was paranoid about that as well and double-checked that the test runs. :D