Fix deferred components spread on asset generator

Author: Ian Maciel

packages/core/lib/generators/assets_generator.dart

@@ -50,10 +50,19 @@ class AssetsGenConfig {
 }
 
 /// Merge the deferred assets with the main assets.
-List<Object> _buildAssetsList(Config config) => [
-      ...config.pubspec.flutter.assets,
-      ...?config.pubspec.flutter.deferredComponents
-    ];
+List<Object> _buildAssetsList(Config config) {
+  // We may have several deferred components, with a list of assets for each.
+  // So before spreading the list of deferred components, we need to spread
+  // the list of assets for each deferred component.
+  final List<Object> deferredAssets = [];
+  config.pubspec.flutter.deferredComponents?.forEach((deferredComponent) {
+    // Include all manipulated assets to the list of deferred assets.
+    deferredAssets.addAll(deferredComponent.assets ?? []);
+  });
+
+  // Merge the deferred assets with the main assets.
+  return [...config.pubspec.flutter.assets, ...deferredAssets];
+}
 
 String generateAssets(
   AssetsGenConfig config,