If I understand correctly, one advantage of il2js should be that all the core .NET libraries won't have to be ported to JS, as the needed library methods get expanded into IL.
In mscorlib however, there are many methods that are implemented directly by the .NET runtime; i.e. they are not implemented in CIL. Cil2Js will provide replacements for these, either written directly in JS, or in CIL (compiled from C# source) that will provide identical functionality.
For example, string.Length is not implemened in CIL. In fact, may of the methods on 'basic' types such as System.String, System.Array, System.Int32, etc... are implemented internally by the .NET runtime.