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/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();
 }