implement msbuild property to give access to internal apis.

packages/Avalonia/Avalonia.targets

@@ -1,3 +1,19 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <Import Project="$(MSBuildThisFileDirectory)\AvaloniaBuildTasks.targets"/>
+
+  <Target Name="RemoveAvaloniaRefAssemblies" AfterTargets="ResolveTargetingPackAssets" Condition="'$(AvaloniaAccessUnstablePrivateApis)'=='True'">
+    <ItemGroup>
+      <ReferencesToRemove Include="@(Reference)" Condition="'%(Reference.NuGetPackageId)' == 'Avalonia'" />
+    </ItemGroup>
+    <ItemGroup>
+      <Reference Remove="@(ReferencesToRemove)" />
+    </ItemGroup>
+  </Target>
+  <Target Name="AddReferencePathsToRealAvaloniaAssemblies" BeforeTargets="CoreCompile" Condition="'$(AvaloniaAccessUnstablePrivateApis)'=='True'">
+    <ItemGroup>
+      <Reference Include="$(MSBuildThisFileDirectory)/lib/net6.0/*.dll"/>
+      <ReferencePath Include="$(MSBuildThisFileDirectory)/lib/net6.0/*.dll"/>
+      <ReferencePathWithRefAssemblies Include="$(MSBuildThisFileDirectory)/lib/net6.0/*.dll"/>
+    </ItemGroup>
+  </Target>
 </Project>