The supercharged U.S. economy, powered by brisk consumer and government spending, grew at a robust 5.4% annual rate during the first three months of 2000, the Commerce Department said today. The increase in the gross domestic product, the nation's total output of goods and services, matched the government's previous estimate and followed the fourth quarter's 7.3% rate of growth, the best showing in nearly 16 years. Many analysts expected the government's second estimate of first-quarter growth, which is based on more data than its initial calculation, would come in at a slightly slower rate of 5.2%.