diff -r 000000000000 -r 6474c204b198 browser/devtools/styleeditor/utils.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/devtools/styleeditor/utils.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,40 @@
+/* -*- Mode: Javascript; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ft=javascript ts=2 et sw=2 tw=80: */
+/* 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/. */
+
+const {Cc, Ci, Cu, Cr} = require("chrome");
+
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/devtools/event-emitter.js");
+
+exports.PREF_ORIG_SOURCES = "devtools.styleeditor.source-maps-enabled";
+
+/**
+ * A PreferenceObserver observes a pref branch for pref changes.
+ * It emits an event for each preference change.
+ */
+function PrefObserver(branchName) {
+  this.branchName = branchName;
+  this.branch = Services.prefs.getBranch(branchName);
+  this.branch.addObserver("", this, false);
+
+  EventEmitter.decorate(this);
+}
+
+exports.PrefObserver = PrefObserver;
+
+PrefObserver.prototype = {
+  observe: function(subject, topic, data) {
+    if (topic == "nsPref:changed") {
+      this.emit(this.branchName + data);
+    }
+  },
+
+  destroy: function() {
+    if (this.branch) {
+      this.branch.removeObserver('', this);
+    }
+  }
+};
\ No newline at end of file