Follow up on #25738 (#26196)

* Follow up on #25738

* PR Comments

Author: OmarTawfik

src/System.Collections.Immutable/src/System.Collections.Immutable.csproj

@@ -9,6 +9,7 @@
     <FileAlignment>512</FileAlignment>
     <DocumentationFile>$(OutputPath)$(MSBuildProjectName).xml</DocumentationFile>
     <GenerateAppxPackageOnBuild>False</GenerateAppxPackageOnBuild>
+    <PackageTargetFramework Condition="'$(TargetGroup)' == 'netstandard1.0'">netstandard1.0;portable-net45+win8+wp8+wpa81</PackageTargetFramework>
   </PropertyGroup>
   <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'netcoreapp-Debug|AnyCPU'" />
   <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'netcoreapp-Release|AnyCPU'" />