_cacheNamespaceLoaderPromise(locale, namespace, promise) {
    if (!this.__namespaceLoaderPromisesCache[locale]) {
      this.__namespaceLoaderPromisesCache[locale] = {};
    }
    this.__namespaceLoaderPromisesCache[locale][namespace] = promise;
  }
@param {string} locale
@param {string} namespace
@param {Promise<Object|void>} promise
@protected