roslyn: MSBuildWorkspace can't compile Class Library referencing PCL

Related to #560 and various dupes #1373, #2700, #1894

We first noticed this problem when trying to compile XUnit with MSBuildWorkspace. All the Desktop projects failed to build but all the PCLs were fine.

To repro:

• Create new ClassLibrary project (I’ve called mine Desktop)
• Create new PortableClassLibrary project (I’m called mine PortableClassLibrary)
• Create reference to PortableClassLibrary in ClassLibrary
• The projects must interact in some way. For example:

In the ClassLibrary I use the following:

namespace Desktop
{
    public class Class1
    {
        public void MyMethod()
        {
            var test = new PortableClassLibrary.Class1();
            var obj = test.PCLMethod();
        }
    }
}

And in the PCL:

namespace PortableClassLibrary
{
    public class Class1
    {
        public object PCLMethod()
        {
            return new object();
        }
    }
}

Now in another solution try to compile the above with Roslyn:

var msws = MSBuildWorkspace.Create(new Dictionary<string, string> { { "CheckForSystemRuntimeDependency", "true" } });
var soln = msws.OpenSolutionAsync(@"C:\Users\Josh\Documents\GitHub\Orange\Desktop\Desktop.sln").Result;
var comp = soln.Projects.First().GetCompilationAsync().Result;
var errs = comp.GetDiagnostics().Where(n => n.Severity == DiagnosticSeverity.Error).ToList();
//One error

Expected: Project builds without errors (as it does in Visual Studio).

Actual: We receive the following error:

C:\Users\Josh\Documents\GitHub\Orange\Desktop\Desktop\Class1.cs(14,23): error CS0012: The type 'Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.