 Copy static assets from .next folder when exporting (#3719)
 Copy static assets from .next folder when exporting (#3719)

* Copy .next/static directory when exporting

* Use nextDir

JeromeFitz/next.js,BlancheXu/test,azukaru/next.js,flybayer/next.js,JeromeFitz/next.js,azukaru/next.js,JeromeFitz/next.js,flybayer/next.js,zeit/next.js,flybayer/next.js,JeromeFitz/next.js,zeit/next.js,zeit/next.js,azukaru/next.js,flybayer/next.js,BlancheXu/test,azukaru/next.js,BlancheXu/test