More AppContext cleanups (#7073)

packages/flutter_tools/lib/src/ios/mac.dart

diff --git a/packages/flutter_tools/lib/src/ios/mac.dart b/packages/flutter_tools/lib/src/ios/mac.dart
index da726d5..f727f38 100644
--- a/packages/flutter_tools/lib/src/ios/mac.dart
+++ b/packages/flutter_tools/lib/src/ios/mac.dart
@@ -56,7 +56,7 @@
   }
 
   /// Returns [XCode] active in the current app context.
-  static XCode get instance => context[XCode] ?? (context[XCode] = new XCode());
+  static XCode get instance => context[XCode];
 
   bool get isInstalledAndMeetsVersionCheck => isInstalled && xcodeVersionSatisfactory;