diff -r 000000000000 -r 6474c204b198 toolkit/modules/debug.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/modules/debug.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,80 @@
+/* vim:set ts=2 sw=2 sts=2 ci et: */
+/* -*- Mode: Java; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+
+/* 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/. */
+
+// This file contains functions that are useful for debugging purposes from
+// within JavaScript code.
+
+this.EXPORTED_SYMBOLS = ["NS_ASSERT"];
+
+var gTraceOnAssert = true;
+
+/**
+ * This function provides a simple assertion function for JavaScript.
+ * If the condition is true, this function will do nothing.  If the
+ * condition is false, then the message will be printed to the console
+ * and an alert will appear showing a stack trace, so that the (alpha
+ * or nightly) user can file a bug containing it.  For future enhancements, 
+ * see bugs 330077 and 330078.
+ *
+ * To suppress the dialogs, you can run with the environment variable
+ * XUL_ASSERT_PROMPT set to 0 (if unset, this defaults to 1).
+ *
+ * @param condition represents the condition that we're asserting to be
+ *                  true when we call this function--should be
+ *                  something that can be evaluated as a boolean.
+ * @param message   a string to be displayed upon failure of the assertion
+ */
+
+this.NS_ASSERT = function NS_ASSERT(condition, message) {
+  if (condition)
+    return;
+
+  var releaseBuild = true;
+  var defB = Components.classes["@mozilla.org/preferences-service;1"]
+                       .getService(Components.interfaces.nsIPrefService)
+                       .getDefaultBranch(null);
+  try {
+    switch (defB.getCharPref("app.update.channel")) {
+      case "nightly":
+      case "aurora":
+      case "alpha":
+      case "beta":
+      case "default":
+        releaseBuild = false;
+    }
+  } catch(ex) {}
+
+  var caller = arguments.callee.caller;
+  var assertionText = "ASSERT: " + message + "\n";
+
+  // Report the error to the console
+  Components.utils.reportError(assertionText);
+
+  if (releaseBuild) {
+    return;
+  }
+
+  // dump the stack to stdout too in non-release builds
+  var stackText = "";
+  if (gTraceOnAssert) {
+    stackText = "Stack Trace: \n";
+    var count = 0;
+    while (caller) {
+      stackText += count++ + ":" + caller.name + "(";
+      for (var i = 0; i < caller.arguments.length; ++i) {
+        var arg = caller.arguments[i];
+        stackText += arg;
+        if (i < caller.arguments.length - 1)
+          stackText += ",";
+      }
+      stackText += ")\n";
+      caller = caller.arguments.callee.caller;
+    }
+  }
+
+  dump(assertionText + stackText);
+}