And also, instead of injecting IApplicationEnvironment you may use PlatformServices.Default.Application.ApplicationBasePath.
EDIT: Here's a possible implementation of MapPath/UnmapPath as extensions to PlatformServices:
removed (see EDIT2)
EDIT2: Slightly modified, IsPathMapped() added as well as some checks to see if path mapping/unmapping is really needed. 
public static class PlatformServicesExtensions
{
    public static string MapPath(this PlatformServices services, string path)
    {
        var result = path ?? string.Empty;
        if (services.IsPathMapped(path) == false)
        {
            var wwwroot = services.WwwRoot();
            if (result.StartsWith("~", StringComparison.Ordinal))
            { 
                result = result.Substring(1); 
            }
            if (result.StartsWith("/", StringComparison.Ordinal))
            { 
                result = result.Substring(1);
            }
            result = Path.Combine(wwwroot, result.Replace('/', '\\'));
        }
        return result;
    }
    public static string UnmapPath(this PlatformServices services, string path)
    {
        var result = path ?? string.Empty;
        if (services.IsPathMapped(path))
        {
            var wwwroot = services.WwwRoot();
            result = result.Remove(0, wwwroot.Length);
            result = result.Replace('\\', '/');
            var prefix = (result.StartsWith("/", StringComparison.Ordinal) ? "~" : "~/");
            result = prefix + result;
        }
        return result;
    }
    public static bool IsPathMapped(this PlatformServices services, string path)
    {
        var result = path ?? string.Empty;
        return result.StartsWith(services.Application.ApplicationBasePath,
            StringComparison.Ordinal);
    }
    public static string WwwRoot(this PlatformServices services)
    {
        // todo: take it from project.json!!!
        var result = Path.Combine(services.Application.ApplicationBasePath, "wwwroot");
        return result;
    }
}
EDIT3: PlatformServices.WwwRoot() return the actual execution path and in .net core 2.0, DEBUG mode it is xxx\bin\Debug\netcoreapp2.0, which, obviously is not what is required. Instead, replace PlatformServices with IHostingEnvironment and use environment.WebRootPath.