Returns cached or newly created instance of a given service.
We cache only default instances (where no locales or options are provided).
function cachedOrNewService(service, locales, options, defaults) {
  var useOptions = (IS_UNDEFINED(defaults)) ? options : defaults;
  if (IS_UNDEFINED(locales) && IS_UNDEFINED(options)) {
    checkDateCacheCurrent();
    if (IS_UNDEFINED(defaultObjects[service])) {
      defaultObjects[service] = new savedObjects[service](locales, useOptions);
    }
    return defaultObjects[service];
  }
  return new savedObjects[service](locales, useOptions);
}