Initializes the given object so it's a valid NumberFormat instance.
Useful for subclassing.
function initializeNumberFormat(numberFormat, locales, options) {
  if (%IsInitializedIntlObject(numberFormat)) {
    throw MakeTypeError(kReinitializeIntl, "NumberFormat");
  }

  if (IS_UNDEFINED(options)) {
    options = {};
  }

  var getOption = getGetOption(options, 'numberformat');

  var locale = resolveLocale('numberformat', locales, options);

  var internalOptions = {};
  defineWEProperty(internalOptions, 'style', getOption(
    'style', 'string', ['decimal', 'percent', 'currency'], 'decimal'));

  var currency = getOption('currency', 'string');
  if (!IS_UNDEFINED(currency) && !isWellFormedCurrencyCode(currency)) {
    throw MakeRangeError(kInvalidCurrencyCode, currency);
  }

  if (internalOptions.style === 'currency' && IS_UNDEFINED(currency)) {
    throw MakeTypeError(kCurrencyCode);
  }

  var currencyDisplay = getOption(
      'currencyDisplay', 'string', ['code', 'symbol', 'name'], 'symbol');
  if (internalOptions.style === 'currency') {
    defineWEProperty(internalOptions, 'currency', %StringToUpperCase(currency));
    defineWEProperty(internalOptions, 'currencyDisplay', currencyDisplay);
  }

  // Digit ranges.
  var mnid = getNumberOption(options, 'minimumIntegerDigits', 1, 21, 1);
  defineWEProperty(internalOptions, 'minimumIntegerDigits', mnid);

  var mnfd = options['minimumFractionDigits'];
  var mxfd = options['maximumFractionDigits'];
  if (!IS_UNDEFINED(mnfd) || internalOptions.style !== 'currency') {
    mnfd = getNumberOption(options, 'minimumFractionDigits', 0, 20, 0);
    defineWEProperty(internalOptions, 'minimumFractionDigits', mnfd);
  }

  if (!IS_UNDEFINED(mxfd) || internalOptions.style !== 'currency') {
    var min_mxfd = internalOptions.style === 'percent' ? 0 : 3;
    mnfd = IS_UNDEFINED(mnfd) ? 0 : mnfd;
    var fallback_limit = (mnfd > min_mxfd) ? mnfd : min_mxfd;
    mxfd = getNumberOption(options, 'maximumFractionDigits', mnfd, 20, fallback_limit);
    defineWEProperty(internalOptions, 'maximumFractionDigits', mxfd);
  }

  var mnsd = options['minimumSignificantDigits'];
  var mxsd = options['maximumSignificantDigits'];
  if (!IS_UNDEFINED(mnsd) || !IS_UNDEFINED(mxsd)) {
    mnsd = getNumberOption(options, 'minimumSignificantDigits', 1, 21, 0);
    defineWEProperty(internalOptions, 'minimumSignificantDigits', mnsd);

    mxsd = getNumberOption(options, 'maximumSignificantDigits', mnsd, 21, 21);
    defineWEProperty(internalOptions, 'maximumSignificantDigits', mxsd);
  }

  // Grouping.
  defineWEProperty(internalOptions, 'useGrouping', getOption(
    'useGrouping', 'boolean', UNDEFINED, true));

  // ICU prefers options to be passed using -u- extension key/values for
  // number format, so we need to build that.
  var extensionMap = parseExtension(locale.extension);

  /**
   * Map of Unicode extensions to option properties, and their values and types,
   * for a number format.
   */
  var NUMBER_FORMAT_KEY_MAP = {
    'nu': {'property': UNDEFINED, 'type': 'string'}
  };

  var extension = setOptions(options, extensionMap, NUMBER_FORMAT_KEY_MAP,
                             getOption, internalOptions);

  var requestedLocale = locale.locale + extension;
  var resolved = %object_define_properties({}, {
    currency: {writable: true},
    currencyDisplay: {writable: true},
    locale: {writable: true},
    maximumFractionDigits: {writable: true},
    minimumFractionDigits: {writable: true},
    minimumIntegerDigits: {writable: true},
    numberingSystem: {writable: true},
    requestedLocale: {value: requestedLocale, writable: true},
    style: {value: internalOptions.style, writable: true},
    useGrouping: {writable: true}
  });
  if (HAS_OWN_PROPERTY(internalOptions, 'minimumSignificantDigits')) {
    defineWEProperty(resolved, 'minimumSignificantDigits', UNDEFINED);
  }
  if (HAS_OWN_PROPERTY(internalOptions, 'maximumSignificantDigits')) {
    defineWEProperty(resolved, 'maximumSignificantDigits', UNDEFINED);
  }
  var formatter = %CreateNumberFormat(requestedLocale,
                                      internalOptions,
                                      resolved);

  if (internalOptions.style === 'currency') {
    %object_define_property(resolved, 'currencyDisplay',
        {value: currencyDisplay, writable: true});
  }

  %MarkAsInitializedIntlObjectOfType(numberFormat, 'numberformat', formatter);
  numberFormat[resolvedSymbol] = resolved;
  if (FLAG_intl_extra) {
    %object_define_property(resolved, 'pattern', patternAccessor);
    %object_define_property(numberFormat, 'resolved', resolvedAccessor);
  }

  return numberFormat;
}