Reland: "Make `PlatformDispatcher.locale` and `locales` return consistent values (#22267)" (#22473)

This reverts commit d37b8621, relanding #22267, now that flutter/flutter#70252 has landed to prevent the problems it had the first time.

Here is the original PR description:

For some reason (probably a bad merge on my part), the return values for locale and locales are not the same on web as they are on other platforms for the PlatformDispatcher and SingletonFlutterWindow classes. The web versions are non-nullable and the others are nullable.

This converts them to be the same (non-nullable) values

lib/ui/platform_dispatcher.dart

@@ -551,12 +551,7 @@ class PlatformDispatcher {
   ///
   /// This is equivalent to `locales.first` and will provide an empty non-null
   /// locale if the [locales] list has not been set or is empty.
-  Locale? get locale {
-    if (locales != null && locales!.isNotEmpty) {
-      return locales!.first;
-    }
-    return null;
-  }
+  Locale get locale => locales.first;
 
   /// The full system-reported supported locales of the device.
   ///
@@ -573,7 +568,7 @@ class PlatformDispatcher {
   ///
   ///  * [WidgetsBindingObserver], for a mechanism at the widgets layer to
   ///    observe when this value changes.
-  List<Locale>? get locales => configuration.locales;
+  List<Locale> get locales => configuration.locales;
 
   /// Performs the platform-native locale resolution.
   ///
@@ -649,12 +644,7 @@ class PlatformDispatcher {
   }
 
   // Called from the engine, via hooks.dart
-  String _localeClosure() {
-    if (locale == null) {
-      return '';
-    }
-    return locale.toString();
-  }
+  String _localeClosure() => locale.toString();
 
   /// The lifecycle state immediately after dart isolate initialization.
   ///

lib/ui/window.dart

@@ -340,7 +340,7 @@ class SingletonFlutterWindow extends FlutterWindow {
   ///
   /// This is equivalent to `locales.first` and will provide an empty non-null
   /// locale if the [locales] list has not been set or is empty.
-  Locale? get locale => platformDispatcher.locale;
+  Locale get locale => platformDispatcher.locale;
 
   /// The full system-reported supported locales of the device.
   ///
@@ -358,7 +358,7 @@ class SingletonFlutterWindow extends FlutterWindow {
   ///
   ///  * [WidgetsBindingObserver], for a mechanism at the widgets layer to
   ///    observe when this value changes.
-  List<Locale>? get locales => platformDispatcher.locales;
+  List<Locale> get locales => platformDispatcher.locales;
 
   /// Performs the platform-native locale resolution.
   ///
@@ -850,11 +850,11 @@ class Window extends SingletonFlutterWindow {
 
   @override
   // ignore: unnecessary_overrides
-  Locale? get locale => super.locale;
+  Locale get locale => super.locale;
 
   @override
   // ignore: unnecessary_overrides
-  List<Locale>? get locales => super.locales;
+  List<Locale> get locales => super.locales;
 
   @override
   // ignore: unnecessary_overrides