China’s imports tumbled in August, adding to concerns about the health of the world’s second-largest economy and its contribution to global growth. Much of China’s imports are commodities and other raw materials going into factories that turn them into goods for sale overseas, so the fall could be an ominous sign for exports in the coming months. Exports fell less than forecast, sliding 5.5 percent, but analysts were still doubtful that China can now achieve its year-end trade growth target of 6 percent.