anchor.diff:                    headerText,
                     style: Theme.of(context)
                         .textTheme
  -                      .bodyText1
  +                      .bodyText1!
                         .copyWith(color: foregroundColor),
                   ),
                   const SizedBox(width: defaultSpacing),
  
positive.code: 
  class BannerMessagesController {
    final _messages = <String, ValueNotifier<List<BannerMessage>>>{};
    final _dismissedMessageKeys = <Key>{};
  
    void addMessage(BannerMessage message) {
      // We push the banner message in a post frame callback because otherwise,
      // we'd be trying to call setState while the parent widget `BannerMessages`
      // is in the middle of `build`.
      WidgetsBinding.instance.addPostFrameCallback((_) {
        if (isMessageDismissed(message) || isMessageVisible(message)) return;
        final messages = _messagesForScreen(message.screenId);
        messages.value.add(message);
  
negative.code:     // We push the banner message in a post frame callback because otherwise,
      // we'd be trying to call setState while the parent widget `BannerMessages`
      // is in the middle of `build`.
      WidgetsBinding.instance!.addPostFrameCallback((_) {
        if (dismiss) {
          assert(!_dismissedMessageKeys.contains(message.key));
          _dismissedMessageKeys.add(message.key);
  