constructor(toolboxItemDef, parentToolbox) {
    super(toolboxItemDef, parentToolbox);
    /**
     * The button element.
     * @type {?HTMLLabelElement}
     */
    this.label = null;
  }
Constructor for a label in the toolbox.
@param {!Blockly.utils.toolbox.ToolboxItemInfo} toolboxItemDef The toolbox
   item definition. This comes directly from the toolbox definition.
@param {!Blockly.IToolbox} parentToolbox The toolbox that holds this
   toolbox item.
@override