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/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;