code: function() {
      this.jsonInit({
        "message0": Blockly.Msg.LISTS_LENGTH_TITLE,
        "args0": [
          {
            "type": "input_value",
            "name": "VALUE",
            "check": ['String', 'Array']
          }
        ],
        "output": 'Number',
        "colour": Blockly.Blocks.lists.HUE,
        "tooltip": Blockly.Msg.LISTS_LENGTH_TOOLTIP,
        "helpUrl": Blockly.Msg.LISTS_LENGTH_HELPURL
      });
    }
docs: Block for list length.
  @this Blockly.Block