In the Roslyn preview that was released at Build 2014 the way references to global assmeblies are added was changed. Before the preview I could use code like this:

var compilation = Compilation.Create(assemblyName, new CompilationOptions(OutputKind.DynamicallyLinkedLibrary))
    .AddReferences(MetadataReference.CreateAssemblyReference("mscorlib"))
    .AddReferences(MetadataReference.CreateAssemblyReference("System"))
    .AddReferences(MetadataReference.CreateAssemblyReference("System.Core"))
    .AddReferences(new MetadataFileReference(this.GetType().Assembly.Location))
    .AddSyntaxTrees(syntaxTree);

The static factory method "MetadataReference.CreateAssemblyReference" added a reference to global assemblies such as "mscorlib.dll" or "System.dll". In the Roslyn preview, the same be achieved like so:

var assemblyPath = Path.GetDirectoryName(typeof(object).Assembly.Location);

var compilation = CSharpCompilation.Create(assemblyName)
    .WithOptions(new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary))
    .AddReferences(new MetadataFileReference(Path.Combine(assemblyPath, "mscorlib.dll")))
    .AddReferences(new MetadataFileReference(Path.Combine(assemblyPath, "System.dll")))
    .AddReferences(new MetadataFileReference(Path.Combine(assemblyPath, "System.Core.dll")))
    .AddReferences(new MetadataFileReference(Assembly.GetEntryAssembly().Location))
    .AddSyntaxTrees(syntaxTree);