Today, Google made J2CL public. The repo can be found here: https://github.com/google/j2cl.
Now, developers can test application and frameworks against J2CL Check, how Nalu (https://github.com/NaluKit/nalu) and MVP4G2 (https://github.com/mvp4g/mvp4g2) will work with J2CL.
A good point to start is using the project generators of MVP4G2 and Nalu:
* Nalu: http://www.mvp4g.org/gwt-boot-starter-nalu/GwtBootStarterNalu.html#application/setUp
* MVP4G2: http://www.mvp4g.org/gwt-boot-starter-mvp4g2/GwtBootStarterMvp4g2.html