If there were multiple or nested scroll views on a given page, the situation
was worsened significantly - each scroll view would dispatch multiple
notifications per frame. For example, in the Dart/Flutter Devtools flamegraph
page, we found that about 30% of CPU time was spent dispatching notifications.

In order to reduce the cost of dispatching notifications, we have changed
notification dispatch so that it only visits ancestors that are notification
listeners, reducing the number of elements visited per frame.

However, the old notification system exposed the fact that it traversed
each element as part of its API via Notification.visitAncestor. This
method is no longer supported as we no longer visit all ancestor elements.

Description of change

Notification.visitAncestor has been removed.
Any classes that extend Notification should
no longer override this method.

* If you don’t implement a custom Notification
that overrides Notification.visitAncestor,
then no changes are required. **