[pigeon] Unify iOS and macOS test plugins (flutter#9100)

Updates the iOS and macOS test plugin harnesses to use shared darwin source support, so that we don't have two copies of everything. This will reduce the complexity of maintaining/updating the test harnesses going forward.

Author: stuartmorgan-g

packages/pigeon/platform_tests/alternate_language_test_plugin/ios/.gitignore renamed to packages/pigeon/platform_tests/alternate_language_test_plugin/darwin/.gitignore

@@ -1,4 +1,4 @@
-# TODO(stuartmorgan): Remove this once more of the files that macOS doesn't
+# TODO(stuartmorgan): Remove this once more of the files that iOS/macOS don't
 # use have been eliminated via test consolidation.
 *.gen.h
 *.gen.m

packages/pigeon/platform_tests/alternate_language_test_plugin/macos/Classes/.gitignore renamed to packages/pigeon/platform_tests/alternate_language_test_plugin/darwin/Classes/.gitignore

@@ -2,7 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#if TARGET_OS_OSX
 #import <FlutterMacOS/FlutterMacOS.h>
+#else
+#import <Flutter/Flutter.h>
+#endif
 
 #import "CoreTests.gen.h"
 

packages/pigeon/platform_tests/alternate_language_test_plugin/macos/Classes/AlternateLanguageTestPlugin.h renamed to packages/pigeon/platform_tests/alternate_language_test_plugin/darwin/Classes/AlternateLanguageTestPlugin.h

@@ -15,13 +15,14 @@ A plugin to test Pigeon generation for secondary languages (e.g., Java, Objectiv
   s.source           = { :http => 'https://github.com/flutter/packages/tree/main/packages/pigeon' }
   s.source_files = 'Classes/**/*'
   s.public_header_files = 'Classes/**/*.h'
-  s.dependency 'Flutter'
-  s.platform = :ios, '12.0'
+  s.ios.dependency 'Flutter'
+  s.osx.dependency 'FlutterMacOS'
+  s.ios.deployment_target = '12.0'
+  s.osx.deployment_target = '10.14'
   s.xcconfig = {
     'LIBRARY_SEARCH_PATHS' => '$(TOOLCHAIN_DIR)/usr/lib/swift/$(PLATFORM_NAME)/ $(SDKROOT)/usr/lib/swift',
     'LD_RUNPATH_SEARCH_PATHS' => '/usr/lib/swift',
   }
-
-  # Flutter.framework does not contain a i386 slice.
-  s.pod_target_xcconfig = { 'DEFINES_MODULE' => 'YES', 'EXCLUDED_ARCHS[sdk=iphonesimulator*]' => 'i386' }
+  s.pod_target_xcconfig = { 'DEFINES_MODULE' => 'YES' }
+  s.swift_version = '5.0'
 end