diff -r 000000000000 -r 6474c204b198 toolkit/components/contentprefs/tests/unit_cps2/AsyncRunner.jsm
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/components/contentprefs/tests/unit_cps2/AsyncRunner.jsm	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,72 @@
+/* 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/. */
+
+let EXPORTED_SYMBOLS = [
+  "AsyncRunner",
+];
+
+const { interfaces: Ci, classes: Cc } = Components;
+
+function AsyncRunner(callbacks) {
+  this._callbacks = callbacks;
+  this._iteratorQueue = [];
+
+  // This catches errors reported to the console, e.g., via Cu.reportError.
+  Cc["@mozilla.org/consoleservice;1"].
+    getService(Ci.nsIConsoleService).
+    registerListener(this);
+}
+
+AsyncRunner.prototype = {
+
+  appendIterator: function AR_appendIterator(iter) {
+    this._iteratorQueue.push(iter);
+  },
+
+  next: function AR_next(/* ... */) {
+    if (!this._iteratorQueue.length) {
+      this.destroy();
+      this._callbacks.done();
+      return;
+    }
+
+    // send() discards all arguments after the first, so there's no choice here
+    // but to send only one argument to the yielder.
+    let args = [arguments.length <= 1 ? arguments[0] : Array.slice(arguments)];
+    try {
+      var val = this._iteratorQueue[0].send.apply(this._iteratorQueue[0], args);
+    }
+    catch (err if err instanceof StopIteration) {
+      this._iteratorQueue.shift();
+      this.next();
+      return;
+    }
+    catch (err) {
+      this._callbacks.error(err);
+    }
+
+    // val is truthy => call next
+    // val is an iterator => prepend it to the queue and start on it
+    if (val) {
+      if (typeof(val) != "boolean")
+        this._iteratorQueue.unshift(val);
+      this.next();
+    }
+  },
+
+  destroy: function AR_destroy() {
+    Cc["@mozilla.org/consoleservice;1"].
+      getService(Ci.nsIConsoleService).
+      unregisterListener(this);
+    this.destroy = function AR_alreadyDestroyed() {};
+  },
+
+  observe: function AR_consoleServiceListener(msg) {
+    if (msg instanceof Ci.nsIScriptError &&
+        !(msg.flags & Ci.nsIScriptError.warningFlag))
+    {
+      this._callbacks.consoleError(msg);
+    }
+  },
+};