Fix Disposable's prototype handling

src/ol/Disposable.js

@@ -1,13 +1,22 @@
 /**
  * @module ol/Disposable
  */
-import {VOID} from './functions.js';
 
 /**
  * @classdesc
  * Objects that need to clean up after themselves.
  */
 class Disposable {
+
+  constructor() {
+    /**
+     * The object has already been disposed.
+     * @type {boolean}
+     * @private
+     */
+    this.disposed_ = false;
+  }
+
   /**
    * Clean up.
    */
@@ -17,19 +26,12 @@ class Disposable {
       this.disposeInternal();
     }
   }
+
+  /**
+   * Extension point for disposable objects.
+   * @protected
+   */
+  disposeInternal() {}
 }
 
-/**
- * The object has already been disposed.
- * @type {boolean}
- * @private
- */
-Disposable.prototype.disposed_ = false;
-
-/**
- * Extension point for disposable objects.
- * @protected
- */
-Disposable.prototype.disposeInternal = VOID;
-
 export default Disposable;