Added MarkupExtension class back to avoid confusion

src/Markup/Avalonia.Markup.Xaml/Avalonia.Markup.Xaml.csproj

@@ -15,6 +15,7 @@
         <Compile Include="Converters\MemberSelectorTypeConverter.cs" />
         <Compile Include="Converters\TimeSpanTypeConverter.cs" />
         <Compile Include="Extensions.cs" />
+        <Compile Include="MarkupExtension.cs" />
         <Compile Include="MarkupExtensions\DynamicResourceExtension.cs" />
         <Compile Include="MarkupExtensions\ResourceInclude.cs" />
         <Compile Include="MarkupExtensions\StaticResourceExtension.cs" />

src/Markup/Avalonia.Markup.Xaml/MarkupExtension.cs

@@ -0,0 +1,9 @@
+using System;
+
+namespace Avalonia.Markup.Xaml
+{
+    public abstract class MarkupExtension
+    {
+        public abstract object ProvideValue(IServiceProvider serviceProvider);
+    }
+}