<PackageReference Include="NETStandard.Library" Version="2.0.0-preview2-25401-01" />

System.Threading.Tasks.TaskExtensions

public static class TaskExtensions
namespace System.Threading.Tasks { public static class TaskExtensions { public static Task Unwrap(this Task<Task> task); public static Task<TResult> Unwrap<TResult>(this Task<Task<TResult>> task); } }