Welcome to Shaun Luttin's public notebook. It contains rough, practical notes. The guiding idea is that, despite what marketing tells us, there are no experts at anything. Sharing our half-baked ideas helps everyone. We're all just muddling thru. Find out more about our work at bigfont.ca.
Short answer: on release of Visual Studio 15.
In the meantime, keep using project.json. MSFT has deprecated project.json and xproj in favor of csproj, in order to "allow unification of the other app models." Support of the new csproj system will coincide with the release of Visual Studio 15 RTM.