code: function assertString(value, name, description) {
    if (!underscore.isString(value)) {
      throw new Error(
          '<' + value + '> is not a string, ' + name + ': ' + description);
    }
  }
docs: ============================================================================== Value Validators ============================================================================== 
  Throws an Error if value isn't a string.
  @param {*} value
  @param {string} name The option name.
  @param {string} description Description of the option.