content model: Avoid `classes.contains`, which does extra work like creating a Set

[chrisbobbe]

Greg and I discovered on our call today that classes.contains does some extra work:

  /// Determine if this element contains the class [value].
  ///
  /// This is the Dart equivalent of jQuery's
  /// [hasClass](http://api.jquery.com/hasClass/).
  @override
  bool contains(Object? value) => readClasses().contains(value);

  @override
  Set<String> readClasses() {
    final s = LinkedHashSet<String>();
    final classname = _element.className;

    for (var name in classname.split(' ')) {
      final trimmed = name.trim();
      if (trimmed.isNotEmpty) {
        s.add(trimmed);
      }
    }
    return s;
  }

When we're checking that there are no classes, we should just check if className is empty. When we're checking that there is one class, we should just do string equality on className. When we're checking that there is more than one class, we might want to do a regular expression or something.