[ios26]fix host engine compile error (flutter#174723)

## What it does

This adds `-plugin-path` flag to `swiftc` cmd to fix a compile error
related to missing swift plugin. The plugin is under
`mac_host_toolchain_path`, which is only available on macs. However, we
only build mac host engine on macs, so we are good.

## About unit test

This swiftc.py file currently has no test coverage, but it should - I
filed flutter#174843. However, given
there's a big fire (flutter#174513)
going on, and we are approaching iOS 26 release, I have to prioritize
that one.

I will manually verify the solution. 

*List which issues are fixed by this PR. You must list at least one
issue. An issue is not required if the PR fixes something trivial like a
typo.*

Fixes flutter#172155


*If you had to change anything in the [flutter/tests] repo, include a
link to the migration guide as per the [breaking change policy].*

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

**Note**: The Flutter team is currently trialing the use of [Gemini Code
Assist for
GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code).
Comments from the `gemini-code-assist` bot should not be taken as
authoritative feedback from the Flutter team. If you find its comments
useful you can update your code accordingly, but if you are unsure or
disagree with the feedback, please feel free to wait for a Flutter team
member's review for guidance on which automated comments should be
addressed.

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: hellohuanlin

engine/src/build/toolchain/darwin/swiftc.py

@@ -444,6 +444,14 @@ def invoke_swift_compiler(args, extras_args, build_cache_dir, output_file_map):
         args.file_prefix_map,
     ])
 
+  # Since iOS/macOS 26, building host engine requires setting -plugin-path for the `testing` plugin under XcodeDefault.xctoolchain.
+  testing_plugin_path = os.path.join(args.mac_host_toolchain_path, 'usr/lib/swift/host/plugins/testing')
+  if os.path.isdir(testing_plugin_path):
+    swiftc_args.extend([
+      '-plugin-path',
+      testing_plugin_path,
+    ])
+
   swift_toolchain_path = args.swift_toolchain_path
   if not swift_toolchain_path:
     swift_toolchain_path = os.path.join(os.path.dirname(args.sdk_path),
@@ -586,6 +594,10 @@ def main(args):
                       default='',
                       help='path to the Swift toolchain to use')
 
+  parser.add_argument('--mac-host-toolchain-path',
+                      default='',
+                      help='path to the mac host toolchain to use')
+
   parser.add_argument('--whole-module-optimization',
                       default=False,
                       action='store_true',

engine/src/build/toolchain/mac/BUILD.gn

@@ -283,6 +283,11 @@ template("mac_toolchain") {
                         rebase_path(swift_toolchain_path, root_build_dir)
       }
 
+      if (mac_host_toolchain_path != "") {
+        _extra_flags += " --mac-host-toolchain-path " +
+                        rebase_path(mac_host_toolchain_path, root_build_dir)
+      }
+
       if (swift_whole_module_optimization) {
         _extra_flags += " --whole-module-optimization"
       }