Revert "Revert "Allow any iOS app to be added to an existing host app (#70647)" (#70739)" (#70740)

This reverts commit aab9a76ee4cd97199e9d21118a0a3cfbda80b899.

packages/flutter_tools/lib/src/project.dart

diff --git a/packages/flutter_tools/lib/src/project.dart b/packages/flutter_tools/lib/src/project.dart
index 9e36c46..1ee6b9d 100644
--- a/packages/flutter_tools/lib/src/project.dart
+++ b/packages/flutter_tools/lib/src/project.dart
@@ -665,15 +665,12 @@
       )
     );
     if (framework.existsSync()) {
-      final Directory engineDest = ephemeralDirectory
-          .childDirectory('Flutter')
-          .childDirectory('engine');
       final File podspec = framework.parent.childFile('Flutter.podspec');
       globals.fsUtils.copyDirectorySync(
         framework,
-        engineDest.childDirectory('Flutter.framework'),
+        engineCopyDirectory.childDirectory('Flutter.framework'),
       );
-      podspec.copySync(engineDest.childFile('Flutter.podspec').path);
+      podspec.copySync(engineCopyDirectory.childFile('Flutter.podspec').path);
     }
   }
 
@@ -704,6 +701,22 @@
         : hostAppRoot.childDirectory(_hostAppProjectName);
   }
 
+  File get pluginRegistrantHeader {
+    final Directory registryDirectory = isModule ? pluginRegistrantHost.childDirectory('Classes') : pluginRegistrantHost;
+    return registryDirectory.childFile('GeneratedPluginRegistrant.h');
+  }
+
+  File get pluginRegistrantImplementation {
+    final Directory registryDirectory = isModule ? pluginRegistrantHost.childDirectory('Classes') : pluginRegistrantHost;
+    return registryDirectory.childFile('GeneratedPluginRegistrant.m');
+  }
+
+  Directory get engineCopyDirectory {
+    return isModule
+        ? ephemeralDirectory.childDirectory('Flutter').childDirectory('engine')
+        : hostAppRoot.childDirectory('Flutter');
+  }
+
   Future<void> _overwriteFromTemplate(String path, Directory target) async {
     final Template template = await Template.fromName(
       path,