We should always be agnostic about the child widget type, otherwise it violates our composition model.

There's basically two principles we can apply here:

• Wrapping a widget X in a widget Y that does nothing but return the widget X should always work exactly the same as using X directly. For example, wrapping a Text in a Tooltip should always work.

• Replacing a widget X with an entirely independent widget Y that happens to be implemented exactly the same way should always work exactly the same as using X directly.

Ideally, anywhere you accept a Widget you should accept a Placeholder or a widget equivalent to Placeholder, and act the same as if you'd gotten anything else.

Places I've noticed where we violate this:

• CupertinoNavigationBar.middle + CupertinoNavigationBar.largeTitle, we assert that middle is Text.
• CupertinoScaffold.tabbed requires a CupertinoTabBar.
• CupertinoScaffold.navigationBar is handled differently if it is a CupertinoNavigationBar

cc @xster @cbracken