Investor fears about slowing economic growth in China, US corporate earnings, and monetary policy here and in Europe resulted in a two-day rout in global markets, capped by a 318-point drop in the Dow Friday. It was the index’s worst day since last June. Despite the sell-off, US stocks remain near record highs after surging 30 percent last year. US stocks have not endured a correction — a drop of 10 percent or more over time — since October 2011. The turbulence coincides with a global economic shift: China and other emerging market economies appear to be hitting trouble just as the US and European economies finally show signs of renewed strength.