Use Server.MapPath when HTTPContext.Current is null.

using System;
using System.IO;
using System.Web.Hosting;

namespace Unicorn.Configuration
{
    internal static class ConfigurationUtility
    {
        /// <summary>
        /// Applies the available formats for a Unicorn configuration path
        /// (i.e. root-relative, absolute) to a given path.
        /// https://github.com/kamsar/Unicorn/blob/master/src/Unicorn/Configuration/ConfigurationUtility.cs
        /// </summary>
        public static string ResolveConfigurationPath(string configPath)
        {
            if (configPath.StartsWith("~/"))
            {
                // +1 to Stack Overflow:
                // http://stackoverflow.com/questions/4742257/how-to-use-server-mappath-when-httpcontext-current-is-nothing

                // Support unit testing scenario where hosting environment is not initialized.
                var hostingRoot = HostingEnvironment.IsHosted
                                  ? HostingEnvironment.MapPath("~/")
                                  : AppDomain.CurrentDomain.BaseDirectory;

                return Path.Combine(hostingRoot, configPath.Substring(2).Replace('/', '\\'));
            }

            return configPath;
        }
    }
}