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_shopper/test/cart_widget_test.dart

@@ -24,7 +24,7 @@ Widget createCartScreen() => MultiProvider(
           },
         ),
       ],
-      child: MaterialApp(
+      child: const MaterialApp(
         home: MyCart(),
       ),
     );

provider_shopper/test/catalog_widget_test.dart

@@ -21,7 +21,7 @@ Widget createCatalogScreen() => MultiProvider(
           },
         ),
       ],
-      child: MaterialApp(
+      child: const MaterialApp(
         home: MyCatalog(),
       ),
     );

provider_shopper/test/login_widget_test.dart

@@ -26,8 +26,8 @@ void main() {
       child: MaterialApp(
         initialRoute: '/',
         routes: {
-          '/': (context) => MyLogin(),
-          '/catalog': (context) => MyCatalog(),
+          '/': (context) => const MyLogin(),
+          '/catalog': (context) => const MyCatalog(),
         },
       ),
     ));

provider_shopper/test/widget_test.dart

@@ -9,7 +9,7 @@ import 'package:provider_shopper/main.dart';
 void main() {
   testWidgets('smoke test', (tester) async {
     // Build our app and trigger a frame.
-    await tester.pumpWidget(MyApp());
+    await tester.pumpWidget(const MyApp());
 
     // Navigating through login page.
     await tester.tap(find.text('ENTER'));