は is used after に to emphasize に, however the sentence you wrote is not grammatically correct. To express the meaning "as for me, I will go to Japan", you could simply say 僕は日本に行く.
A grammatically correct example would be 中国に行くけど、日本には行かない。which would be something like "I'll go to China, but I'm not going to *Japan*/but as for Japan, I'm not going there."