What happens when the face of a country’s anti-corruption movement suddenly is investigated for the sort of bribery he once condemned? Ukraine, a U.S. ally and neighboring foe of Russia, is soon to find out. And it’s a case with implications in the United States, where the fallout from the unproven Trump-Russia collusion scandal has engulfed several Ukrainians. The country’s chief corruption prosecutor on Thursday opened an investigation into “suspicions” that Serhiy Leshchenko, a crusading anti-corruption member of Ukraine’s parliament and former investigative journalist, accepted bribes in 2016 from a Russian source that enabled him to buy a luxury condo...