assertOpen(url, windowName) {
    assertEquals(url, this.openUrl_);
    assertEquals(windowName, this.windowName_);
    this.openUrl_ = null;
    this.windowName_ = null;
  }
Asserts the given URL is loaded in the given window.
@param {string} url The URL to be loaded.
@param {string} windowName The window name to be loaded into.