The “America First” ETFs will aim to turn political uncertainty into an investment theme. - MarketWatch photo illustration/Getty Images From artificial intelligence to clean energy, Wall Street’s collection of thematic exchange-traded funds have been riding the wave of the stock market’s most exciting trends, turning hot ideas into neatly packed investable opportunities. Now, a new set of President Donald Trump–related ETFs will be testing whether political power itself can function as an investable them ...