Move OnExtensionType and fix netstandard error

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlOptionMarkupExtensionTransformer.cs

@@ -55,7 +55,7 @@ internal class AvaloniaXamlIlOptionMarkupExtensionTransformer : IXamlAstTransfor
 
                 var shouldRemoveProp = false;
                 var onObjs = extProp.Values.OfType<XamlAstObjectNode>()
-                    .Where(o => o.Type.GetClrType().GetFqn() == AvaloniaXamlIlWellKnownTypes.OnFqn).ToArray();
+                    .Where(o => o.Type.GetClrType() == context.GetAvaloniaTypes().OnExtensionType).ToArray();
                 if (onObjs.Any())
                 {
                     shouldRemoveProp = true;
@@ -284,7 +284,7 @@ internal class AvaloniaXamlIlOptionMarkupExtensionTransformer : IXamlAstTransfor
             var types = Branches.Select(b => b.Value.Type);
             if (DefaultNode?.Type is { } type)
             {
-                types = types.Append(type);
+                types = types.Concat(new [] { type });
             }
             return types.Select(t => t.GetClrType()).ToArray().GetCommonBaseClass();
         }

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlWellKnownTypes.cs

@@ -9,8 +9,6 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
 {
     class AvaloniaXamlIlWellKnownTypes
     {
-        public const string OnFqn = "Avalonia.Markup.Xaml:Avalonia.Markup.Xaml.MarkupExtensions.On";
-        
         public IXamlType RuntimeHelpers { get; }
         public IXamlType AvaloniaObject { get; }
         public IXamlType IAvaloniaObject { get; }
@@ -34,6 +32,7 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
         public IXamlType DataTypeAttribute { get; }
         public IXamlType MarkupExtensionOptionAttribute { get; }
         public IXamlType MarkupExtensionDefaultOptionAttribute { get; }
+        public IXamlType OnExtensionType { get; }
         public IXamlType UnsetValueType { get; }
         public IXamlType StyledElement { get; }
         public IXamlType IStyledElement { get; }
@@ -134,6 +133,7 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
             DataTypeAttribute = cfg.TypeSystem.GetType("Avalonia.Metadata.DataTypeAttribute");
             MarkupExtensionOptionAttribute = cfg.TypeSystem.GetType("Avalonia.Metadata.MarkupExtensionOptionAttribute");
             MarkupExtensionDefaultOptionAttribute = cfg.TypeSystem.GetType("Avalonia.Metadata.MarkupExtensionDefaultOptionAttribute");
+            OnExtensionType = cfg.TypeSystem.GetType("Avalonia.Markup.Xaml.MarkupExtensions.On");
             AvaloniaObjectBindMethod = AvaloniaObjectExtensions.FindMethod("Bind", IDisposable, false, IAvaloniaObject,
                 AvaloniaProperty,
                 IBinding, cfg.WellKnownTypes.Object);