Setting data source material appearance (flat shaded GeoJsonDataSource?)

I’m trying to flat shade my extruded geojson polygons.
I can loop through the entities and set the color like so:

var dataSource = Cesium.GeoJsonDataSource.load('buildings.geojson').then(
    function(dataSource) {
        var p = dataSource.entities.values;
        for (var i = 0; i < p.length; i++) {
            p[i].polygon.extrudedHeight = -15;
            p[i].polygon.flat = true;
            var colorProperty = Cesium.Color.YELLOW;              
            p[i].polygon.material = new Cesium.ColorMaterialProperty(colorProperty);    

When creating objects manually using Cesium.Primitive it seems you can flat shade by setting the flat property on the appearance as in the Star Burst example:

new Cesium.Primitive({
  geometryInstances: instances,
  appearance: new Cesium.PerInstanceColorAppearance({
    flat: true

Is there a way to do something similar with datasources to set the appearance of the geojson datasource entities to flat shade the polygons?