Json DesignData ignores read only properties.

Question

So the great thing about 8.1 apps is that you can use json in your DesignData, saving lots of time when working with Json api's. While it works great with get/set properties, I'm having trouble getting the design layout to bind to read only properties, despite the class type being set in the definition. It's just blank :(

DesignData :

 d:DataContext="{Binding Source={d:DesignData Source=/SampleData.json, Type=local:MyViewModel}}"

SampleData.json

{
  "FirstName":"John",
  "LastName":"Doe"
}

MyViewModel Class

public class MyViewModel
{
   public string FirstName {get; set;}
   public string LastName {get; set;}
   public string FullName 
   {
     get {
       return FirstName + " " + LastName;
     }
   }
}

Xaml

<TextBlock Text="{Binding FullName}" />

Am I doing something incorrectly?