I’ve then verified the default Universal apps template and found that every project is referencing the “Shared” one using this Import block in the .jsproj file:
<Import Project="..\Shared\<Shared-Project-Name>.projitems" Label="Shared" />
To enable it for your project, just open the “Solution Explorer” in Visual Studio and:
- right-click the project and select “Unload Project“;
- select “Edit <Your-Project-Name>.jsproj“;
- add the “Import Project” block using the code editor and save it;
- right-click and select “Reload Project“.
The shared project code should now be correctly referenced and available in your original app.
Same technique can obviously be used by Windows Phone 8.1 solutions (or other targets / platforms).