A request that the node should be dismissed.
Snackbar, for example, may have a dismiss action to indicate to the
user that it can be removed after it is no longer relevant. On Android,
(with TalkBack) special hint text is spoken when focusing the node and
a custom action is availible in the local context menu. On iOS,
(with VoiceOver) users can perform a standard gesture to dismiss it.
static const SemanticsAction dismiss = const SemanticsAction._(_kDismissIndex)