Make FlutterPluginRegistrant a static framework so add-to-app can use plugins with static frameworks (#40401)

packages/flutter_tools/lib/src/plugins.dart

diff --git a/packages/flutter_tools/lib/src/plugins.dart b/packages/flutter_tools/lib/src/plugins.dart
index 7d8a279..a88e288 100644
--- a/packages/flutter_tools/lib/src/plugins.dart
+++ b/packages/flutter_tools/lib/src/plugins.dart
@@ -383,6 +383,7 @@
   s.source_files =  "Classes", "Classes/**/*.{h,m}"
   s.source           = { :path => '.' }
   s.public_header_files = './Classes/**/*.h'
+  s.static_framework    = true
   s.dependency '{{framework}}'
   {{#plugins}}
   s.dependency '{{name}}'