[JsType(JsMode.Prototype, AutomaticPropertiesAsFields = false)]
public abstract class ValueSet
{
    //[JsProperty(NativeField = false)]
    public abstract JsNumber FieldIndicator { get; set; }

}

[JsType(JsMode.Prototype)]
public class FreeValueSet : ValueSet
{
    public FreeValueSet(JsNumber x)
    {
        X = x;
    }

    JsNumber X;

    public override JsNumber FieldIndicator
    {
        get { return X; }
        set { X = value; }
    }
}

[JsType(JsMode.Prototype)]
public class Class1
{
    public static void Main(String[] args)
    {
        ValueSet vs = new FreeValueSet(2);
        HtmlContext.console.info(vs.FieldIndicator);
    }
}