Fix child caching in opacity_layer (#17914)

Choose a child more likely to remain stable from frame to frame as the target to cache in the OpacityLayer.
https://dart.googlesource.com/external/github.com/flutter/engine/+/efe3f4569a6aa49935ffb9a688812f46a2971b89
1 file changed
tree: 63e7a86e56877affefd6b54e23f3ef6ab5dacf03
  1. commits.json