add previewer location as an msbuild property.

Avalonia.sln

@@ -128,6 +128,7 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "ControlCatalog.NetCore", "s
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Props", "Props", "{F3AC8BC1-27F5-4255-9AFC-04ABFD11683A}"
 	ProjectSection(SolutionItems) = preProject
+		build\Avalonia.Designer.HostApp.props = build\Avalonia.Designer.HostApp.props
 		build\Base.props = build\Base.props
 		build\Binding.props = build\Binding.props
 		build\JetBrains.Annotations.props = build\JetBrains.Annotations.props

build/Avalonia.Designer.HostApp.props

@@ -0,0 +1,8 @@
+<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <PropertyGroup Condition=" $(TargetFramework.StartsWith('netcore')) ">
+    <AvaloniaPreviewerToolPath>$(MSBuildThisFileDirectory)\..\..\tools\netcoreapp2.0\designer\Avalonia.Designer.HostApp.dll</AvaloniaPreviewerToolPath>
+  </PropertyGroup>
+  <PropertyGroup Condition=" !$(TargetFramework.StartsWith('netcore')) ">
+    <AvaloniaPreviewerToolPath>$(MSBuildThisFileDirectory)\..\..\tools\net461\designer\Avalonia.Designer.HostApp.exe</AvaloniaPreviewerToolPath>
+  </PropertyGroup>
+</Project>

packages/Avalonia/Avalonia.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="AggregatePackage.NuGet.Sdk">
+<Project Sdk="AggregatePackage.NuGet.Sdk">
   <PropertyGroup>
       <TargetFrameworks>netstandard2.0;net461;netcoreapp2.0</TargetFrameworks>
   </PropertyGroup>
@@ -27,6 +27,11 @@
         <Visible>false</Visible>
         <BuildAction>None</BuildAction>
       </_PackageFiles>
+      <_PackageFiles Include="../../build/Avalonia.Designer.HostApp.props">
+        <PackagePath>build/Avalonia.props</PackagePath>
+        <Visible>false</Visible>
+        <BuildAction>None</BuildAction>
+      </_PackageFiles>
     </ItemGroup>
   </Target>