Missing implementation of IProvideValueTarget.TargetProperty property (IMarkupExtension)

Unfortunately implementation of IProvideValueTarget.TargetProperty property is missing in Xamarin.Forms.
The implementation of property looks like this:

public object TargetProperty
{
  get { throw new NotImplementedException(); }
  private set { }
}

It's not very convenient for writing custom MarkupExtensions without this property. I use this workaround to get property name:
For Xamarin.Forms < 1.4:

public static class Ext
{
    public static PropertyInfo GetPropertyEx(this Type type, string name)
    {
        while (type != null)
        {
            var typeInfo = type.GetTypeInfo();
            var p = typeInfo.GetDeclaredProperty(name);
            if (p != null)
                return p;
            type = typeInfo.BaseType;
        }
        return null;
    }
}


var property = serviceProvider.GetType().GetPropertyEx("IXamlNodeProvider");
var xamlNodeProvider = property.GetValue(serviceProvider);

property = xamlNodeProvider.GetType().GetPropertyEx("XamlNode");
var xamlNode = property.GetValue(xamlNodeProvider);

property = xamlNode.GetType().GetPropertyEx("Parent");
var parentNode = property.GetValue(xamlNode);

property = parentNode.GetType().GetPropertyEx("Properties");
var properties = (IDictionary)property.GetValue(parentNode);

object xmlName = null;
foreach (DictionaryEntry entry in properties)
{
    if (ReferenceEquals(entry.Value, xamlNode))
    {
        xmlName = entry.Key;
        break;
    }
}

property = xmlName.GetType().GetPropertyEx("LocalName");
var propertyName = (string)property.GetValue(xmlName);

var target = ((IProvideValueTarget)serviceProvider.GetService(typeof(IProvideValueTarget))).TargetObject;
var targetProperty = target.GetType().GetPropertyEx(propertyName);

For Xamarin.Forms >= 1.4:

var provideValueTarget = (IProvideValueTarget) serviceProvider.GetService(typeof (IProvideValueTarget));

var property = provideValueTarget.GetType().GetPropertyEx("Node");
var xamlNode = property.GetValue(provideValueTarget);

property = xamlNode.GetType().GetPropertyEx("Parent");
var parentNode = property.GetValue(xamlNode);

property = parentNode.GetType().GetPropertyEx("Properties");
var properties = (IDictionary)property.GetValue(parentNode);

object xmlName = null;
foreach (DictionaryEntry entry in properties)
{
    if (ReferenceEquals(entry.Value, xamlNode))
    {
        xmlName = entry.Key;
        break;
    }
}

property = xmlName.GetType().GetPropertyEx("LocalName");
var propertyName = (string)property.GetValue(xmlName);

var target = provideValueTarget.TargetObject;
var targetProperty = target.GetType().GetPropertyEx(propertyName);

Dear Xamarin.Forms team please add the implementation of this property, this should not be a problem.

Regards