fix example build (#415)

example/pubspec.yaml

@@ -70,7 +70,7 @@ flutter:
 
   # To add assets to your application, add an assets section, like this:
   assets:
-    - messages.json
+    - assets/messages.json
 
   # An image asset can refer to one or more resolution-specific "variants", see
   # https://flutter.dev/assets-and-images/#resolution-aware