liyuqian 4813cfa506 Ensure OpacityLayer to have a single child (flutter/engine#8362)
It ensures that every OpacityLayer can be optimized by retained
rendering.

Tested with `flutter test --local-engine=host_debug_unopt` in
flutter/flutter/packages/flutter.

Related issues: #21756 #23535
2019-03-29 17:58:30 -07:00
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