Runtime: RC2 netstandard1.5 'Object' se define en un ensamblado al que no se hace referencia.

Tengo una biblioteca de clases en netstandard1.5 , mi project.json está debajo. Cuando ejecuto dotnet build obtengo errores como:

The type 'Exception' is defined in an assembly that is not referenced. You must add a reference to assembly 'mscorlib, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e, Retargetable=Yes'.
The type 'Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'mscorlib, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e, Retargetable=Yes'.
The type 'Uri' is defined in an assembly that is not referenced. You must add a reference to assembly 'System, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e, Retargetable=Yes'.
The type 'Stream' is defined in an assembly that is not referenced. You must add a reference to assembly 'mscorlib, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e, Retargetable=Yes'.

En los paquetes RC1 podría hacer referencia a System.Runtime y esto se arreglaría. Sin embargo, ahora RC2 y hago referencia a NETStandard.Library ese paquete está incluido.

Estoy completamente perplejo en cuanto a por qué recibo estos errores.

proyecto.json

{
    "version": "1.0.0-*",
    "dependencies": {

    },
    "frameworks": {
        "netstandard1.5": {
            "imports": [ "dnxcore50", "portable-net45+win8+wp8+wpa81" ],
            "dependencies": {
                "Enums": { "target": "project" },
                "VQViewModels": { "target": "project" },
                "NovellLdap2": { "target": "project" },
                "DDay": {"target": "project"},

                "NETStandard.Library": "1.5.0-rc2-24027",

                "System.Xml.XmlSerializer": "4.0.11-rc2-24027",
                "System.Data.Common": "4.0.1-rc2-24027",
                "System.Diagnostics.TraceSource": "4.0.0-rc2-24027",
                "System.Net.Requests": "4.0.11-rc2-24027",
                "System.Xml.XmlDocument": "4.0.1-rc2-24027",
                "System.Xml.XPath.XmlDocument": "4.0.1-rc2-24027",
                "System.Collections.Specialized": "4.0.1-rc2-24027",
                "System.Data.SqlClient": "4.1.0-rc2-24027",
                "System.Threading.Thread": "4.0.0-rc2-24027",
                "System.Collections.NonGeneric": "4.0.1-rc2-24027",
                "System.Diagnostics.TextWriterTraceListener": "4.0.0-rc2-24027",
                "Microsoft.AspNetCore.Server.Kestrel": "1.0.0-rc2-final",

                "Dapper": "1.50.0-rc2",
                "FubarCoder.RestSharp.Portable.HttpClient": "3.2.0",
                "MailKit": "1.3.0-beta7",
                "Npgsql": "3.1.0"
            }
        }
    }
}