Isolate selector in footer bar is a PopupMenu instead of a DropdownMenu

[elliette]

Fixes #3985

Also cleans up some unnecessary formatting in the footer (removes individual alignment of items, which was unnecessary because they end up being aligned correctly in their row with spaceBetween).

[kenzieschmoll]

have you checked these changes with a narrow screen width? I suspect that removing the Flexible and Expanded widgets may lead to renderflex overflow errors

[kenzieschmoll]

let's construct this list directly instead of calling children.add a bunch of times:

final children = [
  buildHelpUrlStatus(context, currentScreen, textTheme),
  const BulletSpacer(),
  if (showIsolateSelector)
    ...[
          const IsolateSelector(), 
          const BulletSpacer(),
       ],
   ...
];

We can also just build this directly as the children property in row. It actually makes the build method a little more readable.

Row(
  children: [ ... ],
)

[elliette]

Good point! Only change to your suggestion is I've pulled getting the children into a separate method

[elliette]

have you checked these changes with a narrow screen width? I suspect that removing the Flexible and Expanded widgets may lead to renderflex overflow errors

Good point! Changed so we only show the items that can fit.

Before (without this PR):

Now:

[kenzieschmoll]

write if (!isNarrow) ...[ ... ], instead so you can reuse the if statement

[kenzieschmoll]

one comment then lgtm