Fix `FetchInstructions` assertion (#9)

diff --git a/packages/flutter_image/lib/network.dart b/packages/flutter_image/lib/network.dart
index d66ac52..08b9342 100644
--- a/packages/flutter_image/lib/network.dart
+++ b/packages/flutter_image/lib/network.dart
@@ -97,7 +97,7 @@
         }
       }
       return true;
-    });
+    }());
   }
 
   Future<ImageInfo> _loadWithRetry(NetworkImageWithRetry key) async {