_getUnanimousScriptParentDir = function() {
    var i, jsDir, jsPath, scripts = _document.getElementsByTagName("script");
    for (i = scripts.length; i--; ) {
      if (!(jsPath = scripts[i].src)) {
        jsDir = null;
        break;
      }
      jsPath = _getDirPathOfUrl(jsPath);
      if (jsDir == null) {
        jsDir = jsPath;
      } else if (jsDir !== jsPath) {
        jsDir = null;
        break;
      }
    }
    return jsDir || undefined;
  }
Get the unanimous parent directory of ALL script tags.
If any script tags are either (a) inline or (b) from differing parent
directories, this method must return `undefined`.

@returns String or `undefined`
@private