Enforce use_key_in_widget_constructors and file_names lints (#913)

* Start enforcing use_key_in_widget_constructors and file_names lints

* dart format

* analysis fixes

* analysis fixes, pt2

* analysis fixes, part 3

* Revert platform_design (test failure)

* More reverts

* Notate why we aren't enforcing a lint

platform_channels/lib/src/add_pet_details.dart

@@ -10,6 +10,8 @@ import 'package:platform_channels/src/pet_list_message_channel.dart';
 /// The widget uses [TextField] and [RadioListTile] to take the [PetDetails.breed] and
 /// [PetDetails.petType] from the user respectively.
 class AddPetDetails extends StatefulWidget {
+  const AddPetDetails({Key? key}) : super(key: key);
+
   @override
   _AddPetDetailsState createState() => _AddPetDetailsState();
 }