get closestBlock() {
    var i = this.stack.length
      , block;
    while (i--) {
      block = this.stack[i].block;
      if (block.node) {
        switch (block.node.nodeName) {
          case 'group':
          case 'keyframes':
          case 'atrule':
          case 'atblock':
          case 'media':
          case 'call':
            return block;
        }
      }
    }
  }
Return the closest mixin-able `Block`.

@return {Block}
@api private