Right before Congress passed the Tax Cuts and Jobs Act in December 2017, President Trump proclaimed: “It’ll be fantastic for the middle-income people and for jobs, most of all ... I think we could go to 4%, 5% or even 6% [GDP growth], ultimately. We are back. We are really going to start to rock.” A year later, it’s very clear that the tax cuts boosted gross domestic product and jobs a bit — and just for one year. Its effects are fading as U.S. GDP growth appears likely to weaken in 2019. The only thing that “rocked” were corporate...