“Émigré,” a new oratorio about Jewish refugees who fled Nazi Germany for Shanghai in the late 1930s, begins with a song by two brothers, Josef and Otto, as their steamship approaches a Chinese harbor. “Shanghai, beacon of light on a silent shore,” they sing. “Shanghai, answer these desperate cries.” The emigration of thousands of Central European and Eastern European Jews to China in the late 1930s and early 1940s — and their survival of the Holocaust — is one of World War II’s most dramatic but little-known chapters. In “Émigré,”…