get hasOnlyProperties() {
    if (!this.block) return false;

    var nodes = this.block.nodes;
    for (var i = 0, len = nodes.length; i < len; ++i) {
      var nodeName = nodes[i].nodeName;
      switch (nodes[i].nodeName) {
        case 'property':
        case 'expression':
        case 'comment':
          continue;
        default:
          return false;
      }
    }
    return true;
  }
Check if at-rule's block has only properties.

@return {Boolean}
@api public