Warren Buffett, the 90-year-old “Oracle of Omaha,” remains a firm believer in the American dream, saying in his closely watched annual shareholder letter to “never bet against America.” “In its brief 232 years of existence ... there has been no incubator for unleashing human potential like America,” the chairman and CEO of Berkshire Hathaway wrote in the letter released Saturday. “Despite some severe interruptions, our country’s economic progress has been breathtaking. Our unwavering conclusion: Never bet against America.” Buffett shared a fact in the letter to illustrate Berkshire’s American credentials. He said the conglomerate owns the biggest amount of U.S....