Make upgradeTo an object prototype

Change-Id: Ibdedbc5a57215151f97c5903ac69ba3ce6bab10c
diff --git a/dev/js/src/container/containeritem.js b/dev/js/src/container/containeritem.js
index 10709b8..a9a209b 100644
--- a/dev/js/src/container/containeritem.js
+++ b/dev/js/src/container/containeritem.js
@@ -18,19 +18,6 @@
   },
 
   /**
-   * Upgrade this object to another object,
-   * while private data stays intact.
-   *
-   * @param {Object} An object with properties.
-   */
-  upgradeTo : function (props) {
-    for (let prop in props) {
-      this[prop] = props[prop];
-    };
-    return this;
-  },
-
-  /**
    * Check or set if the item is active
    *
    * @param {boolean|null} State of activity
@@ -105,4 +92,4 @@
     return this._menu;
   }
 
-});
\ No newline at end of file
+});