Actually, Philipp Dolder, one of the contributors to FakeItEasy has come up with an interesting and working approach based on Portable Class Libraries.
http://www.planetgeek.ch/2013/02/01/fakeiteasy-and-windows-store-apps-are-becoming-friends/
In essence, he proposes the following solution:
- Put all your productive code that will be TDD-ed into a PCL class library where you select whatever target frameworks you like to support
- Create your test assembly as a regular class library
- Add the references to FakeItEasy and your test framework(s) of choice (e.g. NUnit + FluentAssertions) to your *.Test assembly
- Only put the UI stuff that you can’t test into a Windows Store App project