fix ConcatPlugin for karma.
fix ConcatPlugin for karma.

use emitAsset instead of writing directly to the filesystem; directory
might not have been created yet; better to let webpack handle it.

LiquidPL/osu-web,nanaya/osu-web,omkelderman/osu-web,notbakaneko/osu-web,notbakaneko/osu-web,omkelderman/osu-web,omkelderman/osu-web,nanaya/osu-web,ppy/osu-web,LiquidPL/osu-web,ppy/osu-web,notbakaneko/osu-web,nanaya/osu-web,LiquidPL/osu-web,LiquidPL/osu-web,nanaya/osu-web,LiquidPL/osu-web,notbakaneko/osu-web,ppy/osu-web,ppy/osu-web,ppy/osu-web,omkelderman/osu-web,notbakaneko/osu-web,nanaya/osu-web