fixes https://github.com/flutter/flutter/issues/115898

`hasChild` checks `_idToChild` map.  `_idToChild` is not assigned until `_callPerformLayout` is called.
So `hasChild` shouldn't be called in `getSize`.

https://github.com/flutter/flutter/blob/df789c9e76098e82f80f2c5de8b5560f360afa40/packages/flutter/lib/src/rendering/custom_layout.dart#L400-L404

Updated docs and example class names.