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/lib/main.dart

@@ -12,10 +12,12 @@ import 'package:provider_shopper/screens/catalog.dart';
 import 'package:provider_shopper/screens/login.dart';
 
 void main() {
-  runApp(MyApp());
+  runApp(const MyApp());
 }
 
 class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   @override
   Widget build(BuildContext context) {
     // Using MultiProvider is convenient when providing multiple objects.
@@ -41,9 +43,9 @@ class MyApp extends StatelessWidget {
         theme: appTheme,
         initialRoute: '/',
         routes: {
-          '/': (context) => MyLogin(),
-          '/catalog': (context) => MyCatalog(),
-          '/cart': (context) => MyCart(),
+          '/': (context) => const MyLogin(),
+          '/catalog': (context) => const MyCatalog(),
+          '/cart': (context) => const MyCart(),
         },
       ),
     );

provider_shopper/lib/screens/cart.dart

@@ -7,6 +7,8 @@ import 'package:provider/provider.dart';
 import 'package:provider_shopper/models/cart.dart';
 
 class MyCart extends StatelessWidget {
+  const MyCart({Key? key}) : super(key: key);
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(

provider_shopper/lib/screens/catalog.dart

@@ -8,6 +8,8 @@ import 'package:provider_shopper/models/cart.dart';
 import 'package:provider_shopper/models/catalog.dart';
 
 class MyCatalog extends StatelessWidget {
+  const MyCatalog({Key? key}) : super(key: key);
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(

provider_shopper/lib/screens/login.dart

@@ -5,6 +5,8 @@
 import 'package:flutter/material.dart';
 
 class MyLogin extends StatelessWidget {
+  const MyLogin({Key? key}) : super(key: key);
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(