Returns a String value representing the result of calling ToNumber(value)
according to the effective locale and the formatting options of this
NumberFormat.
function formatNumber(formatter, value) {
  // Spec treats -0 and +0 as 0.
  var number = TO_NUMBER(value) + 0;

  return %InternalNumberFormat(%GetImplFromInitializedIntlObject(formatter),
                               number);
}