Clustering for Polygons (and polylines, etc) Not Supported

Looks like clustering is only supported for **billboards, points** and **labels** in 1.27.

> new Cesium.DataSource() DataSources/DataSource.js 20
> ... snip
> clustering : EntityCluster DataSources/DataSource.js 96
>
> Gets or sets the clustering options for this data source. This object can be shared between multiple data > sources.
http://cesiumjs.org/Cesium/Build/Documentation/DataSource.html#clustering

> new Cesium.EntityCluster(options) DataSources/EntityCluster.js 72
>
> Defines how screen space objects (billboards, points, labels) are clustered.
> http://cesiumjs.org/Cesium/Build/Documentation/EntityCluster.html

Attempting to cluster polygons doesn't seem to be doing anything:
https://sandcastle.cesium.com/?src=Hello%20World.html&label=Showcases&gist=5a8bc388758fb201c63b2403b58b81bc