Since the Cultural Revolution ended in the mid-1970s, China’s economy, fueled by market reforms, has notched up more than four decades of unbroken gains, enlarging the domestic economy by roughly a hundredfold and transforming the world. That winning streak is over. China on Friday reported a 6.8% year-over-year contraction in its economy for the first three months of the year—the first quarterly decline in gross domestic product since official record-keeping began in 1992 and likely the first since Mao Zedong’s death…