Use case

the form widget can be a powerful tool as it caches and can interact with all its child FormField without having to manually track them through keys. this is useful for a couple build in features like submitting & saving. but blocks the user from creating it's own implementations like for example #107530

Proposal

make the FormState _fields value into a public field so we can use it to extend its functionality

/// State associated with a [Form] widget.
///
/// A [FormState] object can be used to [save], [reset], and [validate] every
/// [FormField] that is a descendant of the associated [Form].
///
/// Typically obtained via [Form.of].
class FormState extends State<Form> {

  final Set<FormFieldState<dynamic>> fields = <FormFieldState<dynamic>>{};
  
   ...

}

this issue would allow us to implement our own extensions that would, for example, make #107528 doable by the user.