add InheritedWidget macro example and fix up FunctionalWidget

[jakemac53]

This would reduce the "Scoped Data Propagation" example here to the following (when used with FunctionalWidget):

@InheritedWidget()
class CustomInheritedWidget {
  final String data;
}

@FunctionalWidget()
Parent _parent(BuildContext context, String data) => CustomInheritedWidget(
      data: data,
      child: Intermediate(),
    );

@FunctionalWidget()
Intermediate _intermediate(BuildContext context) => Child();

@FunctionalWidget()
Child _child(BuildContext context) {
  final data = CustomInheritedWidget.of(context).data;
  return Text("Received data: $data");
}

// Usage
Parent(data: "Some data")

I also cleaned up FunctinalWidget a bit while I was at it.

cc @goderbauer wdyt about a macro like this?

[jakemac53]

also cc @mit-mit

[munificent]

Nit: Looks like this hasn't been run through dart format?

(I noticed the lack of space before .... This is what working on the formatter has done to me. :) )

...Oh, I just realized you can't because of the macro keywords. :(

[jakemac53]

Yeah because dart format doesn't allow the macro keyword so its a pain to use it lol.

[jakemac53]

Fwiw, we should have a larger discussion, but the current way the formatter works with experiments is causing other problems with deprecating old experiments. We should re-evaluate the whole thing and come up with a way to explicitly enable any experiments imo.