Central banks around the world are selling U.S. government bonds at the fastest pace on record, the most dramatic shift in the $12.8 trillion Treasury market since the financial crisis. Sales by China, Russia, Brazil and Taiwan are the latest sign of an emerging-markets slowdown that is threatening to spill over into the U.S. economy. Previously, all four were large purchasers of U.S. debt. Few analysts expect much higher yields in the Treasury market as a result. Foreign private purchases of U.S. debt have increased amid pessimism about the world economic outlook. U.S. firms and financial institutions continue to buy...