Suppress warnings about missing DNXCore reference assemblies

MSBuild currently produces a warning for any target framework that
doesn't have a directory containing its reference assemblies, but
DNXCore50 purposfully has no reference assemblies and currently has no
empty directory to indicate so. Issue #5213 details figuring out what
the proper solution is here, but in the meantime this silences the
warning by pretending that the TargetFrameworkDirectory for DNXCore50 is
.NETPortable, 5.0.

build/Targets/VSL.Imports.targets

@@ -59,15 +59,26 @@
       <RemoveAssemblyFoldersIfNoTargetFramework>false</RemoveAssemblyFoldersIfNoTargetFramework>
   </PropertyGroup>
 
+  <PropertyGroup Condition="'$(TargetFrameworkIdentifier)' == 'DNXCore'">
+    <_TargetFrameworkDirectories>$([System.String]::Join(";", $([Microsoft.Build.Utilities.ToolLocationHelper]::GetPathToReferenceAssemblies(".NETPortable", "v5.0", ""))))</_TargetFrameworkDirectories>
+    <_FullFrameworkReferenceAssemblyPaths>$(_TargetFrameworkDirectories)</_FullFrameworkReferenceAssemblyPaths>
+  </PropertyGroup>
+
   <PropertyGroup Condition="'$(TargetFrameworkIdentifier)' == 'DNXCore'">
       <BaseNuGetRuntimeIdentifier Condition="'$(OS)' == 'Windows_NT'">win</BaseNuGetRuntimeIdentifier>
       <BaseNuGetRuntimeIdentifier Condition="'$(OS)' == 'Unix'">linux</BaseNuGetRuntimeIdentifier>
   </PropertyGroup>
 
-  <Import Project="$(MSBuildExtensionsPath32)\Microsoft\Portable\$(TargetFrameworkVersion)\Microsoft.Portable.VisualBasic.targets" Condition="'$(ProjectLanguage)' == 'VB' And '$(TargetFrameworkIdentifier)' == '.NETPortable'"/>
-  <Import Project="$(MSBuildExtensionsPath32)\Microsoft\Portable\$(TargetFrameworkVersion)\Microsoft.Portable.CSharp.targets" Condition="'$(ProjectLanguage)' == 'CSharp' And '$(TargetFrameworkIdentifier)' == '.NETPortable'"/>
-  <Import Project="$(MSBuildToolsPath)\Microsoft.VisualBasic.targets" Condition="'$(ProjectLanguage)' == 'VB' And '$(TargetFrameworkIdentifier)' != '.NETPortable'" />
-  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" Condition="'$(ProjectLanguage)' == 'CSharp' And '$(TargetFrameworkIdentifier)' != '.NETPortable'" />
+  <Import Project="$(MSBuildExtensionsPath32)\Microsoft\Portable\$(TargetFrameworkVersion)\Microsoft.Portable.VisualBasic.targets"
+          Condition="'$(ProjectLanguage)' == 'VB' And '$(TargetFrameworkIdentifier)' == '.NETPortable'"/>
+  <Import Project="$(MSBuildExtensionsPath32)\Microsoft\Portable\$(TargetFrameworkVersion)\Microsoft.Portable.CSharp.targets"
+          Condition="'$(ProjectLanguage)' == 'CSharp' And '$(TargetFrameworkIdentifier)' == '.NETPortable'"/>
+
+  <Import Project="$(MSBuildToolsPath)\Microsoft.VisualBasic.targets"
+          Condition="'$(ProjectLanguage)' == 'VB' And '$(TargetFrameworkIdentifier)' != '.NETPortable'" />
+  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets"
+          Condition="'$(ProjectLanguage)' == 'CSharp' And '$(TargetFrameworkIdentifier)' != '.NETPortable'" />
+
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" Condition="'$(ProjectLanguage)' == 'C++'" />
 
   <!-- On Mono on *nix the NuGet targets aren't imported by default, so we do that here -->