verifiedLocale method

String verifiedLocale (String newLocale, Function localeExists, { Function onFailure: _throwLocaleError })

Given newLocale return a locale that we have data for that is similar to it, if possible.

If newLocale is found directly, return it. If it can't be found, look up based on just the language (e.g. 'en_CA' -> 'en'). Also accepts '-' as a separator and changes it into '_' for lookup, and changes the country to uppercase.

There is a special case that if a locale named "fallback" is present and has been initialized, this will return that name. This can be useful for messages where you don't want to just use the text from the original source code, but wish to have a universal fallback translation.

Note that null is interpreted as meaning the default locale, so if newLocale is null the default locale will be returned.

Implementation 

static String verifiedLocale(String newLocale, Function localeExists,
    {Function onFailure: _throwLocaleError}) {
  // TODO(alanknight): Previously we kept a single verified locale on the Intl
  // object, but with different verification for different uses, that's more
  // difficult. As a result, we call this more often. Consider keeping
  // verified locales for each purpose if it turns out to be a performance
  // issue.
  if (newLocale == null) {
    return verifiedLocale(getCurrentLocale(), localeExists,
        onFailure: onFailure);
  }
  if (localeExists(newLocale)) {
    return newLocale;
  }
  for (var each in [
    canonicalizedLocale(newLocale),
    shortLocale(newLocale),
    "fallback"
  ]) {
    if (localeExists(each)) {
      return each;
    }
  }
  return onFailure(newLocale);
}