flutter pub upgrade (#178)

provider_counter/lib/main.dart

@@ -17,7 +17,7 @@ void main() {
       // Initialize the model in the builder. That way, Provider
       // can own Counter's lifecycle, making sure to call `dispose`
       // when not needed anymore.
-      builder: (context) => Counter(),
+      create: (context) => Counter(),
       child: MyApp(),
     ),
   );