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();
 }

platform_channels/lib/src/event_channel_demo.dart

@@ -14,6 +14,8 @@ import 'package:platform_channels/src/accelerometer_event_channel.dart';
 /// [Text] widgets to display the value of [AccelerometerReadings.x],
 /// [AccelerometerReadings.y], and [AccelerometerReadings.z] respectively.
 class EventChannelDemo extends StatelessWidget {
+  const EventChannelDemo({Key? key}) : super(key: key);
+
   @override
   Widget build(BuildContext context) {
     final textStyle = Theme.of(context).textTheme.headline5;

platform_channels/lib/src/method_channel_demo.dart

@@ -10,6 +10,8 @@ import 'package:platform_channels/src/counter_method_channel.dart';
 /// It has two [ElevatedButton]s to increment and decrement the value of
 /// [count], and a [Text] widget to display its value.
 class MethodChannelDemo extends StatefulWidget {
+  const MethodChannelDemo({Key? key}) : super(key: key);
+
   @override
   _MethodChannelDemoState createState() => _MethodChannelDemoState();
 }

platform_channels/lib/src/pet_list_screen.dart

@@ -9,6 +9,8 @@ import 'package:platform_channels/src/pet_list_message_channel.dart';
 /// Demonstrates how to use [BasicMessageChannel] to send & receive the platform
 /// Message.
 class PetListScreen extends StatefulWidget {
+  const PetListScreen({Key? key}) : super(key: key);
+
   @override
   _PetListScreenState createState() => _PetListScreenState();
 }
@@ -59,7 +61,7 @@ class _PetListScreenState extends State<PetListScreen> {
 class BuildPetList extends StatelessWidget {
   final List<PetDetails> petList;
 
-  const BuildPetList(this.petList);
+  const BuildPetList(this.petList, {Key? key}) : super(key: key);
 
   @override
   Widget build(BuildContext context) {

platform_channels/lib/src/platform_image_demo.dart

@@ -13,6 +13,8 @@ import 'package:platform_channels/src/image_basic_message_channel.dart';
 /// The widget uses [Image.memory] to display the image obtained from the
 /// platform.
 class PlatformImageDemo extends StatefulWidget {
+  const PlatformImageDemo({Key? key}) : super(key: key);
+
   @override
   _PlatformImageDemoState createState() => _PlatformImageDemoState();
 }