code: public StorageTier getTier(String tierAlias) {
      StorageTier tier = mAliasToTiers.get(tierAlias);
      if (tier == null) {
        throw new IllegalArgumentException(
            ExceptionMessage.TIER_ALIAS_NOT_FOUND.getMessage(tierAlias));
      }
      return tier;
    }
docs: Gets the {@link StorageTier} given its tierAlias. Throws an {@link IllegalArgumentException} if
  the tierAlias is not found.
  
  @param tierAlias the alias of this tier
  @return the {@link StorageTier} object associated with the alias