When you install the Windows Phone SDK this file can be found in C:\Program Files (x86)\Microsoft SDKs\Windows Phone\v7.1\Libraries\Silverlight however that doesn't make it very friendly when people open it without the SDK or you want everything in one place so this simple Nuget package solves that.
Install-Package WindowsPhoneControlLibrary -Version 1.0.0
dotnet add package WindowsPhoneControlLibrary --version 1.0.0
<PackageReference Include="WindowsPhoneControlLibrary" Version="1.0.0" />
paket add WindowsPhoneControlLibrary --version 1.0.0
1.0.0 - First release.
This package has no dependencies.
This package is not used by any NuGet packages.
This package is not used by any popular GitHub repositories.