Use Microsoft.NETFramework.ReferenceAssemblies.

build/NetFX.props

@@ -1,11 +1,7 @@
-<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+<Project>
-  <PropertyGroup Condition=" '$(TargetFramework)' == 'net461' and '$(OS)' == 'Unix' ">
+
-    <FrameworkPathOverride>/usr/lib/mono/4.6.1-api</FrameworkPathOverride>
+  <ItemGroup>
-    <FrameworkPathOverride Condition="$([MSBuild]::IsOsPlatform('OSX'))">/Library/Frameworks/Mono.framework/Versions/Current/lib/mono/4.6.1-api</FrameworkPathOverride>
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0-preview.2" PrivateAssets="All" />
-  </PropertyGroup>
+  </ItemGroup>
-  <PropertyGroup Condition=" '$(TargetFramework)' == 'net47' and '$(OS)' == 'Unix' ">
-    <FrameworkPathOverride>/usr/lib/mono/4.7-api/</FrameworkPathOverride>
-    <FrameworkPathOverride Condition="$([MSBuild]::IsOsPlatform('OSX'))">/Library/Frameworks/Mono.framework/Versions/Current/lib/mono/4.7-api</FrameworkPathOverride>
-  </PropertyGroup>
 
 </Project>