docs: function buildPath(path, parameters) {
    let pathParameters = path.match(/\/:(\w+)\b/g);
    if (pathParameters) {
      for (let i = 0; i < pathParameters.length; ++i) {
        let key = pathParameters[i].substring(2);  // Trim the /:
        if (key in parameters) {
          let value = parameters[key];
          if (WebElement.isId(value)) {
            // When inserting a WebElement into the URL, only use its ID value,
            // not the full JSON.
            value = WebElement.extractId(value);
          }
          path = path.replace(pathParameters[i], '/' + value);
          delete parameters[key];
        } else {
          throw new error.InvalidArgumentError(
              'Missing required parameter: ' + key);
        }
      }
    }
    return path;
  }