Fixes to get Avalonia.Behaviors working again

build/Markup.props

@@ -9,5 +9,6 @@
     <PackageReference Include="System.ComponentModel.Primitives" Version="4.3.0" />
     <PackageReference Include="System.Runtime.Serialization.Primitives" Version="4.3.0" />
     <PackageReference Include="System.Xml.XmlDocument" Version="4.3.0" />
+    <PackageReference Include="System.Xml.ReaderWriter" Version="4.3.0" />
   </ItemGroup>
 </Project>

packages.cake

@@ -248,7 +248,7 @@ public class Packages
                 }
                 .Deps(new string[]{null, "netcoreapp1.0"},
                     "System.ValueTuple", "System.ComponentModel.TypeConverter", "System.ComponentModel.Primitives",
-                    "System.Runtime.Serialization.Primitives", "System.Xml.XmlDocument")
+                    "System.Runtime.Serialization.Primitives", "System.Xml.XmlDocument", "System.Xml.ReaderWriter")
                 .ToArray(),
                 Files = coreLibrariesNuSpecContent
                     .Concat(win32CoreLibrariesNuSpecContent).Concat(net45RuntimePlatform)

src/Markup/Avalonia.Markup.Xaml/PortableXaml/AvaloniaXamlType.cs

@@ -331,7 +331,7 @@ namespace Avalonia.Markup.Xaml.PortableXaml
 
             public override object GetValue(object instance)
             {
-                if (Property != null)
+                if (Property != null && !Property.IsAttached)
                 {
                     return ((IAvaloniaObject)instance).GetValue(Property);
                 }

src/Markup/Avalonia.Markup.Xaml/PortableXaml/portable.xaml.github

@@ -1 +1 @@
-Subproject commit dc333ff66eb1fdd8565e9b831718791675ec569b
+Subproject commit 926e82551c10591f2fa3afb25121ac2d16b272c6