Switched back to #if since ReferenceOutputAssembly doesn't work properly for classic project format

src/Avalonia.Base/Avalonia.Base.csproj

@@ -7,9 +7,7 @@
     <IsPackable>false</IsPackable>
   </PropertyGroup>
   <ItemGroup>
-    <ProjectReference Include="..\Avalonia.Build.Tasks\Avalonia.Build.Tasks.csproj">
+    <ProjectReference Include="..\Avalonia.Build.Tasks\Avalonia.Build.Tasks.csproj"/>
-      <ReferenceOutputAssembly>false</ReferenceOutputAssembly>
-    </ProjectReference>
   </ItemGroup>
   <Import Project="..\..\build\Base.props" />
   <Import Project="..\..\build\Binding.props" />

src/Avalonia.Base/Utilities/AvaloniaResourcesIndex.cs

@@ -9,7 +9,10 @@ using System.Runtime.Serialization.Json;
 
 namespace Avalonia.Utilities
 {
-    public static class AvaloniaResourcesIndexReaderWriter
+    #if !BUILDTASK
+    public
+    #endif
+    static class AvaloniaResourcesIndexReaderWriter
     {
         private const int LastKnownVersion = 1;
         public static List<AvaloniaResourcesIndexEntry> Read(Stream stream)
@@ -34,14 +37,20 @@ namespace Avalonia.Utilities
     }
 
     [DataContract]
-    public class AvaloniaResourcesIndex
+#if !BUILDTASK
+    public
+#endif
+    class AvaloniaResourcesIndex
     {       
         [DataMember]
         public List<AvaloniaResourcesIndexEntry> Entries { get; set; } = new List<AvaloniaResourcesIndexEntry>();
     }
 
     [DataContract]
-    public class AvaloniaResourcesIndexEntry
+#if !BUILDTASK
+    public
+#endif
+    class AvaloniaResourcesIndexEntry
     {
         [DataMember]
         public string Path { get; set; }