Four chemical companies based in China and eight Chinese nationals have been charged with trafficking chemicals used by Mexican drug cartels to manufacture vast quantities of fentanyl later sold in the United States, federal officials said on Friday. The officials said that two of the Chinese defendants — the principal executive and marketing manager of one of the companies — had been arrested recently overseas and taken to Hawaii for a court appearance, and that they would be brought to Manhattan to face prosecution. The new indictments unsealed on Friday…