Returns the current document base URL.
@returns {string}
function getBaseUrl() {
  if (window.document.baseURI) {
    return window.document.baseURI;
  }

  // `document.baseURI` is available everywhere except IE
  if (!baseUrlParsingNode) {
    baseUrlParsingNode = window.document.createElement('a');
    baseUrlParsingNode.href = '.';

    // Work-around for IE bug described in Implementation Notes. The fix in `urlResolve()` is not
    // suitable here because we need to track changes to the base URL.
    baseUrlParsingNode = baseUrlParsingNode.cloneNode(false);
  }
  return baseUrlParsingNode.href;
}