message: Add support for spacers in dropdown component.
diff: @@ -109,10 +109,16 @@ export class Dropdown extends DropdownMixin {
  </button>;
  }
  - const children = _.map(items, (html, key) => {
  + const spacerBefore = this.props.spacerBefore || new Set();
  +
  + const children = [];
  + _.each(items, (html, key) => {
  const klass = classNames({'dropdown__selected': !noSelection && key === selectedKey});
  const onClick_ = this.onClick_.bind(this, key);
  - return <li className={klass} key={key}><a onClick={onClick_}>{html}</a></li>;
  + if (spacerBefore.has(key)) {
  + children.push(<li key={`${key}-spacer`}><div className="divider"></div></li>);
  + }
  + children.push(<li className={klass} key={key}><a onClick={onClick_}>{html}</a></li>);
  });
  return (
  