testGet() {
    const el = dom.createElement(TagName.DIV);
    assertArrayEquals([], classes.get(el));
    el.className = 'C';
    assertArrayEquals(['C'], classes.get(el));
    el.className = 'C D';
    assertArrayEquals(['C', 'D'], classes.get(el));
    el.className = 'C\nD';
    assertArrayEquals(['C', 'D'], classes.get(el));
    el.className = ' C ';
    assertArrayEquals(['C'], classes.get(el));
  }
@fileoverview Shared code for classes_test.html & classes_quirks_test.html.