aSource.activeSourceBuffers.addEventListener("removesourcebuffer", test.unreached_func(
                        "Unexpected removal from activeSourceBuffers list"));
                      mediaSource.activeSourceBuffers.addEventListener("addsourcebuffer", test.unreached_func(
                        "Unexpected insertion in activeSourceBuffers list"));

                      // Changes should only trigger events at the
                      // AudioTrack/VideoTrack instance
                      test.expect