China may follow the USA model. The J20 is it's F22, which will not be exported even to Pakistan. China will develop a smaller less advanced JXX that it probably will export in the future to Pakistan, maybe 2020-30, depending on Pakistan's needs. China would have no need to supply Pakistan with stealth fighters unless India obtains them as well. Therefore, if India gets a stealth fighter by 2020, which is doubtful, then China may export it's F35 version to Pakistan.
The relationship between Pakistan and China revolves around India. If India procures X, China will help Pakistan obtain Xb. If India does not procure X, then there's no need for China to aid Pakistan wntetith Xb.