href(item) {
      const href = item.href;
      let attr;
      if (href) {
        if (attr = this._hrefs && this._hrefs[href]) {
          return attr;
        } else {
          this.sanitizeURL(href).then(attr => {
            // rewrite to use xlink namespace
            attr['xlink:href'] = attr.href;
            attr.href = null;
            (this._hrefs || (this._hrefs = {}))[href] = attr;
          });
        }
      }
      return null;
    }
Get href attributes for a hyperlinked mark item.
@param {Item} item - The mark item.