xamarin.forms.map

public class App : Application
{
    public App ()
    {
        // The root page of your application
        var map = new Map (MapSpan.FromCenterAndRadius (new Position (37, -122), Distance.FromMiles (10)));

        var pin = new Pin () {
            Position = new Position (37, -122),
            Label = "Some Pin!"
        };
        map.Pins.Add (pin);

        var cp = new ContentPage { 
            Content = map, 
        };          
        MainPage = cp;
    }

    protected override void OnStart ()
    {
        // Handle when your app starts
    }

    protected override void OnSleep ()
    {
        // Handle when your app sleeps
    }

    protected override void OnResume ()
    {
        // Handle when your app resumes
    }
}

error
Could not load file or assembly 'Xamarin.Forms.Maps, Version=1.4.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies.