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/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();
 }