When I changed how routes complete their futures, I broke the Dropdown
button because it was still waiting for its own Completer to complete
instead of using the Future returned by push. This patch fixes that
issue.

I've also removed the previous behavior of the DropdownButton forwarding
its text style to its route. The mechansim that we were using doesn't
work properly in all cases. For example, if the DropdownButton is a
child of a LayoutBuilder, then the route will have already built by the
time the DropdownButton gets a chance to forward its text style, causing
an assert in setState.

Finally, I've tweaked PopupMenuButton to work the same way as
DropdownButton in a couple corner cases (e.g., not calling the changed
callback if the button was removed from the tree before the menu
completed its Future).

Fixes #6352