Correct the return type of addRetained (#9025)

It should return void as SceneBuilder::addRetained is returning void.
Thanks yjbanov@ for finding this typo!
https://dart.googlesource.com/external/github.com/flutter/engine/+/3d2e9b24aa712ba60206e7ac392a709df0939635
1 file changed
tree: f08a10859ba019c27be1198bbd4f9919fed57dcf
  1. commits.json