System.IO.Compression not working in JS?

I’m using JS, and my code goes as follows:

import System.IO;
import System.IO.Compression;
private var archive : ZipArchive;

The problem is that just defining the archive variable sets off “The name ‘ZipArchive’ does not denote a valid type (‘not found’).”
I’ve spent quite a while using C# rather than JS, but I think I’m doing the basics right… is there something preventing System.IO.Compression working that I’ve missed?

This is in Unity 4.1.5f1.

Edit: For reference, the MSDN reference of System.IO.Compression

Edit 2: I’m including System.IO AND IO.Compression because the script also requires the streamreader, which is in IO. I showed it here in case it’s what’s causing this problem - I wouldn’t know if it was.

Because its not supported, the it i’m referring to ZipArchive class:

Mono Compatibility