 new layer. This is purely
an optimization. It has no effect on the correctness of rendering.</p>
<p>Passing a layer to <a href="dart-ui/SceneBuilder/addRetained.html">addRetained</a> or as <code>oldLayer</code> argument to a push
method counts as <em>usage</em>. A layer can be used no more than once in a scene.
For example, it may not be passed simultaneously to two push methods, or
to a push method and to <code>addRetained</code>.</p>
<p>When a layer is passed to <a href="dart-ui/SceneBuilder/addRetained.