related flutter/flutter#100624

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Add a new `RouteName` semantic role for nodes with `namesRoute` set without a `scopesRoute`. Such nodes provide a description for the nearest ancestor dialog node. The web equivalent of this is when an element `role="dialog"` is described by pointing to one of its children using `aria-labelledby` and `aria-describedby`. Here's an example:

```
<div
  role="dialog"
  aria-labelledby="dialog1Title"
  aria-describedby="dialog1Desc">
  <h2 id="dialog1Title">Your personal details were successfully updated</h2>
  <p id="dialog1Desc">
    You can change your details at any time in the user account section.
  </p>
  <button>Close</button>
</div>
```

([Source](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/dialog_role))

Flutter currently does not distinguish between "labelled by" and "described by". In my testing, only `aria-describedby` resulted in an announcement of the dialog's description upon opening. `aria-labelledby` required that the dialog element itself be focusable, which is not the case. So I went with `aria-describedby`.

Fixes flutter/flutter#126030