testEnterAtBeginningOfLink() {
    if (userAgent.GECKO) {
      field1.setSafeHtml(false, SafeHtml.create('a', {'href': '/'}, [
        'b',
        SafeHtml.BR,
      ]));
      const helper = new TestHelper(field1.getElement());
      field1.focusAndPlaceCursorAtStart();
      events.fireKeySequence(field1.getElement(), KeyCodes.ENTER);
      helper.assertHtmlMatches('<p>&nbsp;</p><p><a href="/">b<br></a></p>');
    }
  }
Verifies
TagOnEnterHandler.prototype.handleRegularEnterGecko_
when we explicitly split anchor elements. This test runs only for Gecko
since this is a Gecko-only codepath.