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

provider_counter/test/widget_test.dart

@@ -13,7 +13,7 @@ void main() {
     await tester.pumpWidget(
       ChangeNotifierProvider(
         create: (context) => Counter(),
-        child: MyApp(),
+        child: const MyApp(),
       ),
     );