function getElementNamespaceURI(element) {
  return genericPropertyGet(
      Methods.NAMESPACE_URI_GETTER, element, 'namespaceURI',
      function(namespaceURI) {
        return typeof namespaceURI == 'string';
      });
}
Returns an element's namespace URI without falling prey to things like
<form><input name="namespaceURI"></form>.
@param {!Element} element
@return {string}