It's now the law of conservation of energy and mass. They're equivalent. Normally energy doesn't get converted into mass, or mass into energy, but it can happen. At usual energies, the law of conservation of energy, or the law of conservation of mass still hold, just as do Newton's laws.

In fact, some go so far as to say that you don't really increase the mass of an object by speeding it up, but that the energy itself has mass.