There are many ways to measure the health of a country. One way is growth of gross domestic product. Another is economic productivity. But there’s a less conventional measure worth watching: Do the world’s most talented and ambitious people want to immigrate here?Throughout most of America’s history, the answer has been an emphatic yes. Scientists, engineers, entrepreneurs, artists, and strivers from around the world have chosen the United States because they believed they could build something, discover something, or start a business while making a better life in this country. We should worry about the day when this is no…