Replace instanceof checks with other logic

src/ol/source/Cluster.js

@@ -5,6 +5,7 @@
 import {getUid} from '../util.js';
 import {assert} from '../asserts.js';
 import Feature from '../Feature.js';
+import GeometryType from '../geom/GeometryType.js';
 import {scale as scaleCoordinate, add as addCoordinate} from '../coordinate.js';
 import {listen} from '../events.js';
 import EventType from '../events/EventType.js';
@@ -76,7 +77,7 @@ class Cluster extends VectorSource {
      */
     this.geometryFunction = options.geometryFunction || function(feature) {
       const geometry = /** @type {Point} */ (feature.getGeometry());
-      assert(geometry instanceof Point,
+      assert(geometry.getType() == GeometryType.POINT,
         10); // The default `geometryFunction` can only handle `Point` geometries
       return geometry;
     };