VS2017 support (e.g. new csproj format)

[colltoaction]

Just dropping this here so we have this in mind.

Tried using the automatic migration with my project but I'm having some issues. Bug report here: dotnet/msbuild#1354.

[LunicLynx]

Possible solution here: https://github.com/dotnet/cli/issues/4785

[SteveSandersonMS]

This is now published. If you get the latest generator-aspnetcore-spa Yeoman generator, then whenever you create a new project, you'll be able to choose between VS2015-style project.json or VS2017-style .csproj.

If you have an existing project.json project and want to start using VS2017RC, you most likely can just drop in a .csproj produced by the new template and delete your old project.json and .xproj files.