Dedupe message-list pages on opening a notification

[gnprice]

Fixes #1852.

This required a bit of setup in order to get our hands on the right information about the current navigation state.

There were a few product choices to make about the details of the behavior. Quoting from the last commit:

    if (…) {
      // The current page is already a MessageListPage at the desired narrow.
      // Instead of pushing another copy of it, stay there; see #1852.

      // Do dismiss any non-page routes, like dialogs and bottom sheets, though.
      // That way we're presenting the page directly, like the user asked for
      // by opening the notification.
      navigator.popUntil((route) => route is PageRoute);

      // TODO(#1565): Scroll to the specific message if nearby; else jump there,
      //   or push a new page anchored there.
      return;
    }

Selected commit messages

783e1d1 nav: Track the whole navigation stack

Kind of silly that Navigator upstream doesn't provide this directly
in its API. But here it is.

bc396ff nav: Add currentPageRoute getter on NavigationStack

1345d0d nav: Add WidgetRoute.pageElement

5d0703b msglist: Add currentNarrow and stateOfRoute static methods

9e33d49 notif: Dedupe msglist page on opening notif

Fixes #1852.

[rajveermalviya]

Thanks @gnprice! LGTM and tests great. Please go ahead and merge.

(This'll probably create conflicts for #2043, I'll rebase that PR once this is merged.)

[gnprice]

Thanks for the review and testing!