The Prius, the world's first mass-produced hybrid vehicle, has been a sales success in Japan since its debut last December, leading its manufacturer, Toyota Motor Corp., to hope it can break through U.S. consumer's resistance to electric cars when it appears here in 2000. The Prius combines a gasoline engine with an electric motor, promising pollution levels nearly as low as those of fully electric vehicles but with the everyday practicality of a conventional gasoline powered automobile.