Issue 2444 - Make preconfigurable property optional

lib/prefs.js

 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 Cu.import("resource://gre/modules/Services.jsm");
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 
 let {addonRoot, addonName} = require("info");
 let branchName = "extensions." + addonName + ".";
 let branch = Services.prefs.getBranch(branchName);
 let preconfiguredBranch =
     Services.prefs.getBranch(branchName + "preconfigured.");
 let ignorePrefChanges = false;
 
 function init()
 {
   // Load default preferences and set up properties for them
   let defaultBranch = Services.prefs.getDefaultBranch(branchName);
 
   let request = new XMLHttpRequest();
   request.open("GET", addonRoot + "defaults/prefs.json", false);
   request.responseType = "json";
   request.send();
 
   let defaults = request.response.defaults;
+  let preconfigurable = new Set(request.response.preconfigurable);
   for (let pref in defaults)
   {
     let value = defaults[pref];
     let [getter, setter] = typeMap[typeof value];
-    if ("preconfigurable" in request.response &&
-        request.response.preconfigurable.indexOf(pref) != -1)
+    if (preconfigurable.has(pref))
     {
       try
       {
         value = getter(preconfiguredBranch, pref);
       }
       catch (e) {}
     }
     setter(defaultBranch, pref, value);
     defineProperty(pref, false, getter, setter);
   }
@@ skipping 147 matching lines @@
 {
   let str = Cc["@mozilla.org/supports-string;1"].createInstance(Ci.nsISupportsString);
   str.data = newValue;
   branch.setComplexValue(pref, Ci.nsISupportsString, str);
 }
 
 function getJSONPref(branch, pref) JSON.parse(getCharPref(branch, pref))
 function setJSONPref(branch, pref, newValue) setCharPref(branch, pref, JSON.stringify(newValue))
 
 init();