code: function() {
      var OPERATORS =
          [[Blockly.Msg.CONTROLS_WHILEUNTIL_OPERATOR_WHILE, 'WHILE'],
           [Blockly.Msg.CONTROLS_WHILEUNTIL_OPERATOR_UNTIL, 'UNTIL']];
      this.setHelpUrl(Blockly.Msg.CONTROLS_WHILEUNTIL_HELPURL);
      this.setColour(Blockly.Blocks.loops.HUE);
      this.appendValueInput('BOOL')
          .setCheck('Boolean')
          .appendField(new Blockly.FieldDropdown(OPERATORS), 'MODE');
      this.appendStatementInput('DO')
          .appendField(Blockly.Msg.CONTROLS_WHILEUNTIL_INPUT_DO);
      this.setPreviousStatement(true);
      this.setNextStatement(true);
      // Assign 'this' to a variable for use in the tooltip closure below.
      var thisBlock = this;
      this.setTooltip(function() {
        var op = thisBlock.getFieldValue('MODE');
        var TOOLTIPS = {
          'WHILE': Blockly.Msg.CONTROLS_WHILEUNTIL_TOOLTIP_WHILE,
          'UNTIL': Blockly.Msg.CONTROLS_WHILEUNTIL_TOOLTIP_UNTIL
        };
        return TOOLTIPS[op];
      });
    }
docs: Block for 'do while/until' loop.
  @this Blockly.Block