Another FYI - In Europe (e.g., Germany and France) a similar thing has happened. The difference there is a noticeable percentage of the immigrants are Muslim (and are generally less inclinced to assimilate). I remember reading a couple of years ago that soon enough (read: 25 to 50 years?) - if current trends continue - Muslims will be the majority in Germany. So while Germ's economy has fared well as of late, it might be sitting on a cultural powder-keg if the economy stagnates.

As for Japan's economic stagnation of the last couple decades? Relatively low immigration.