The ASP.NET 5 project has no "Suppress Warnings" option in its project property sheet.
Recently I encountered a warning MSB3274 ( dnx451 actually means .net 4.5 ) and I tried to find a way to suppress this warning.
I looked https://github.com/aspnet/Home/wiki/Project.json-file but found nothing related to suppress warnings.
This is not yet supported, the DNX team will try to add it in beta 6: https://github.com/aspnet/dnx/issues/2042.