See https://github.com/dotnet/source-build/issues/286

diff -ruN dotnet/targets/corefx.targets dotnet/targets/corefx.targets
--- dotnet/targets/corefx.targets	2017-10-25 20:40:18.000000000 -0400
+++ dotnet/targets/corefx.targets	2017-11-10 12:39:18.045215772 -0500
@@ -16,7 +16,7 @@
 
   <Target Name="BuildNetStandardLibraryNetFramework"
           BeforeTargets="Package">
-    <Exec Command="$(ProjectDirectory)/build$(ShellExtension) -$(Configuration) -Framework=netfx -OSGroup=Windows_NT -- /p:ILLinkTrimAssembly=false"
+    <Exec Command="$(ProjectDirectory)/build$(ShellExtension) -$(Configuration) -Framework=netfx -OSGroup=Windows_NT -portable=false -- /p:ILLinkTrimAssembly=false"
           WorkingDirectory="$(ProjectDirectory)"
           EnvironmentVariables="@(EnvironmentVariables)" />
   </Target>