code: function() {
      this.jsonInit({
        "message0": "%1",
        "args0": [
          {
            "type": "field_numberdropdown",
            "name": "INDEX",
            "value": "1",
            "min": 1,
            "precision": 1,
            "options": [
              ["1", "1"],
              [Blockly.Msg.DATA_INDEX_LAST, "last"],
              [Blockly.Msg.DATA_INDEX_RANDOM, "random"]
            ]
          }
        ],
        "category": Blockly.Categories.data,
        "extensions": ["colours_textfield", "output_string"]
      });
    }
docs: List index menu, with random option.
  @this Blockly.Block