Make plugins Swift-first on macOS (#33997)

Instead of sharing the iOS codepath that uses an ObjC generated plugin
registrant and expecting plugins to have an ObjC interface layer, switch
to generating a Swift registrant and expecting plugins to have a Swift
interface.

This means plugins on macOS that use Swift won't need an ObjC wrapper,
and plugins that use ObjC will need a Swift wrapper (inverting the
structure relative to iOS).

packages/flutter_tools/lib/src/plugins.dart

@@ -182,7 +182,7 @@ Future<void> _writeAndroidPluginRegistrant(FlutterProject project, List<Plugin>
   _renderTemplateToFile(_androidPluginRegistryTemplate, context, registryPath);
 }
 
-const String _cocoaPluginRegistryHeaderTemplate = '''//
+const String _objcPluginRegistryHeaderTemplate = '''//
 //  Generated file. Do not edit.
 //
 
@@ -198,7 +198,7 @@ const String _cocoaPluginRegistryHeaderTemplate = '''//
 #endif /* GeneratedPluginRegistrant_h */
 ''';
 
-const String _cocoaPluginRegistryImplementationTemplate = '''//
+const String _objcPluginRegistryImplementationTemplate = '''//
 //  Generated file. Do not edit.
 //
 
@@ -218,6 +218,23 @@ const String _cocoaPluginRegistryImplementationTemplate = '''//
 @end
 ''';
 
+const String _swiftPluginRegistryTemplate = '''//
+//  Generated file. Do not edit.
+//
+import Foundation
+import {{framework}}
+
+{{#plugins}}
+import {{name}}
+{{/plugins}}
+
+func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
+  {{#plugins}}
+  {{class}}.register(with: registry.registrar(forPlugin: "{{class}}"))
+{{/plugins}}
+}
+''';
+
 const String _pluginRegistrantPodspecTemplate = '''
 #
 # Generated file, do not edit.
@@ -259,63 +276,59 @@ Future<void> _writeIOSPluginRegistrant(FlutterProject project, List<Plugin> plug
     'plugins': iosPlugins,
   };
   final String registryDirectory = project.ios.pluginRegistrantHost.path;
-  return await _writeCocoaPluginRegistrant(project, context, registryDirectory);
-}
-
-Future<void> _writeMacOSPluginRegistrant(FlutterProject project, List<Plugin> plugins) async {
-  // TODO(stuartmorgan): Replace macosPrefix check with formal metadata check,
-  // see https://github.com/flutter/flutter/issues/33597.
-  final List<Map<String, dynamic>> macosPlugins = plugins
-      .where((Plugin p) => p.pluginClass != null && p.macosPrefix != null)
-      .map<Map<String, dynamic>>((Plugin p) => <String, dynamic>{
-    'name': p.name,
-    'prefix': p.macosPrefix,
-    'class': p.pluginClass,
-  }).toList();
-  final Map<String, dynamic> context = <String, dynamic>{
-    'os': 'macos',
-    'deploymentTarget': '10.13',
-    'framework': 'FlutterMacOS',
-    'plugins': macosPlugins,
-  };
-  final String registryDirectory = project.macos.managedDirectory.path;
-  return await _writeCocoaPluginRegistrant(project, context, registryDirectory);
-}
-
-Future<void> _writeCocoaPluginRegistrant(FlutterProject project,
-    Map<String, dynamic> templateContext, String registryDirectory) async {
-
   if (project.isModule) {
     final String registryClassesDirectory = fs.path.join(registryDirectory, 'Classes');
     _renderTemplateToFile(
       _pluginRegistrantPodspecTemplate,
-      templateContext,
+      context,
       fs.path.join(registryDirectory, 'FlutterPluginRegistrant.podspec'),
     );
     _renderTemplateToFile(
-      _cocoaPluginRegistryHeaderTemplate,
-      templateContext,
+      _objcPluginRegistryHeaderTemplate,
+      context,
       fs.path.join(registryClassesDirectory, 'GeneratedPluginRegistrant.h'),
     );
     _renderTemplateToFile(
-      _cocoaPluginRegistryImplementationTemplate,
-      templateContext,
+      _objcPluginRegistryImplementationTemplate,
+      context,
       fs.path.join(registryClassesDirectory, 'GeneratedPluginRegistrant.m'),
     );
   } else {
     _renderTemplateToFile(
-      _cocoaPluginRegistryHeaderTemplate,
-      templateContext,
+      _objcPluginRegistryHeaderTemplate,
+      context,
       fs.path.join(registryDirectory, 'GeneratedPluginRegistrant.h'),
     );
     _renderTemplateToFile(
-      _cocoaPluginRegistryImplementationTemplate,
-      templateContext,
+      _objcPluginRegistryImplementationTemplate,
+      context,
       fs.path.join(registryDirectory, 'GeneratedPluginRegistrant.m'),
     );
   }
 }
 
+Future<void> _writeMacOSPluginRegistrant(FlutterProject project, List<Plugin> plugins) async {
+  // TODO(stuartmorgan): Replace macosPrefix check with formal metadata check,
+  // see https://github.com/flutter/flutter/issues/33597.
+  final List<Map<String, dynamic>> macosPlugins = plugins
+      .where((Plugin p) => p.pluginClass != null && p.macosPrefix != null)
+      .map<Map<String, dynamic>>((Plugin p) => <String, dynamic>{
+    'name': p.name,
+    'class': p.pluginClass,
+  }).toList();
+  final Map<String, dynamic> context = <String, dynamic>{
+    'os': 'macos',
+    'framework': 'FlutterMacOS',
+    'plugins': macosPlugins,
+  };
+  final String registryDirectory = project.macos.managedDirectory.path;
+  _renderTemplateToFile(
+    _swiftPluginRegistryTemplate,
+    context,
+    fs.path.join(registryDirectory, 'GeneratedPluginRegistrant.swift'),
+  );
+}
+
 /// Rewrites the `.flutter-plugins` file of [project] based on the plugin
 /// dependencies declared in `pubspec.yaml`.
 ///