Help convertings c# to js line --using Callback=System.Action

I am trying to convert a CS script into Unityscript and there are a few lines which I don’t understand how to convert. Could someone tell me if these are convertible?

using Callback = System.Action<string> 
and  
(int)(pos.x - rect.x) 

   and

[DllImport("WebView")] for using osx plugin
    DllImport("__Internal")] for using iphone plugin

You could use:

     System.Action.<String>

in place of Callback

And use

  Mathf.RoundToInt(pos.x - rect.x)