Stocks and bonds tumbled Friday, with the Dow industrials and S&P 500 posting their biggest percentage losses since the Brexit selloff. Fresh signs that central banks could be backing away from easy-money policies helped boost the dollar, while investors sold shares of dividend payers like utilities and telecommunications companies that have been popular with income-seeking investors while rates have been low. Yields on some government bonds reached their highest levels since late June. The Dow Jones Industrial Average fell 394.46 points, or 2.1%, to 18085.45. The S&P 500 declined 2.45%, its biggest drop since it fell 1.8% on June 27, the second day of a selloff that followed the U.K.’s vote to leave the European Union. The Nasdaq Composite lost 2.5%. Trading volumes were elevated, traders said, something that typically suggests conviction in the market’s move.