static getAttributeNS(elem, ns, name) {
    const schemaNS = shaka.util.TXml.getKnownNameSpace(ns);
    // Think this is equivalent
    const attribute = elem.attributes[`${schemaNS}:${name}`];
    return attribute || null;
  }
Gets a namespace-qualified attribute.
@param {!shaka.extern.xml.Node} elem The element to get from.
@param {string} ns The namespace URI.
@param {string} name The local name of the attribute.
@return {?string} The attribute's value, or null if not present.