Use XMLHttpRequest constructor in JS modules

lib/io.js

Index: lib/io.js
===================================================================
--- a/lib/io.js
+++ b/lib/io.js
@@ -67,17 +67,17 @@ let IO = exports.IO =
    * The callback will be called when the operation is done.
    */
   readFromFile: function(/**nsIFile|nsIURI*/ file, /**Boolean*/ decode, /**Object*/ listener, /**Function*/ callback, /**String*/ timeLineID)
   {
     try
     {
       let buffer = "";
       let uri = file instanceof Ci.nsIFile ? Services.io.newFileURI(file) : file;
-      let request = Cc["@mozilla.org/xmlextras/xmlhttprequest;1"].createInstance(Ci.nsIXMLHttpRequest);
+      let request = new XMLHttpRequest();
       request.mozBackgroundRequest = true;
       request.open("GET", uri.spec);
       request.responseType = "moz-chunked-text";
       request.overrideMimeType("text/plain" + (decode ? "? charset=utf-8" : ""));
 
       request.addEventListener("progress", function(event)
       {
         if (timeLineID)