createTreeWalker(filter) {
    return new YXmlTreeWalker(this, filter);
  }
Create a subtree of childNodes.

@example
const walker = elem.createTreeWalker(dom => dom.nodeName === 'div')
for (let node in walker) {
  // `node` is a div node
  nop(node)
}

@param {function(AbstractType<any>):boolean} filter Function that is called on each child element and
                         returns a Boolean indicating whether the child
                         is to be included in the subtree.
@return {YXmlTreeWalker} A subtree and a position within it.

@public