fixup! Add support for Flutter deferred components

Author: Ian Maciel

packages/core/lib/generators/assets_generator.dart

@@ -52,7 +52,7 @@ class AssetsGenConfig {
 /// Merge the deferred assets with the main assets.
 List<Object> _buildAssetsList(Config config) => [
       ...config.pubspec.flutter.assets,
-      ...config.pubspec.flutter.deferredComponents ?? []
+      ...?config.pubspec.flutter.deferredComponents
     ];
 
 String generateAssets(

packages/core/lib/settings/pubspec.dart

@@ -248,8 +248,9 @@ class FlutterGenElementFontsOutputs extends FlutterGenElementOutputs {
       _$FlutterGenElementFontsOutputsFromJson(json);
 }
 
+@JsonSerializable()
 class FlutterDeferredComponents {
-  FlutterDeferredComponents({
+  const FlutterDeferredComponents({
     required this.name,
     required this.assets,
   });