--- a/dom/animation/test/testcommon.js
+++ b/dom/animation/test/testcommon.js
@@ -181,8 +181,28 @@ if (opener) {
 
   function done() {
     opener.add_completion_callback(function() {
       self.close();
     });
     opener.done();
   }
 }
+
+/**
+ * Return a new MutaionObserver which started observing |target| element
+ * with { animations: true, subtree: |subtree| } option.
+ * NOTE: This observer should be used only with takeRecords(). If any of
+ * MutationRecords are observed in the callback of the MutationObserver,
+ * it will raise an assertion.
+ */
+function setupSynchronousObserver(t, target, subtree) {
+   var observer = new MutationObserver(records => {
+     assert_unreached("Any MutationRecords should not be observed in this " +
+                      "callback");
+   });
+  t.add_cleanup(() => {
+    observer.disconnect();
+  });
+  observer.observe(target, { animations: true, subtree: subtree });
+  return observer;
+}
+