The newly updated lint will soon flag for-each in collections.

See discussion: https://github.com/dart-lang/linter/pull/4383

/cc @goderbauer

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.

Rename Sample classes

Remove 1745 decorative breaks

This adds a simple example to CustomMultiChildLayout that lays out colored containers in a cascade.