getI18nBundleSync = packageName => {
    if (I18nBundleInstances.has(packageName)) {
      return I18nBundleInstances.get(packageName);
    }
    const i18nBundle = new I18nBundle(packageName);
    I18nBundleInstances.set(packageName, i18nBundle);
    return i18nBundle;
  }
Returns the I18nBundle instance for the given package synchronously.

@public
@param packageName
@returns { I18nBundle }