[ios26]fix host engine compile error (#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 https://github.com/flutter/flutter/issues/174843. However, given
there's a big fire (https://github.com/flutter/flutter/issues/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 https://github.com/flutter/flutter/issues/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].*

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we expect every widget to implement].
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above.
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- [ ] I added new tests to check the change I am making, or this PR is
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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"
       }