“ Our observations in the former case as to the effect of subsequent legislation in conflict with the stipulations of a treaty are therefore inapplicable to the present case. But all other considerations as to specific exemptions in return for special concessions remain, in answer to the alleged contention of the plaintiffs that articles, the produce and manufacture of the island of San Domingo should be admitted free of duty because similar articles, the produce and manufacture of the Hawaiian islands are thus admitted. ”