Fix PlatformDispatcher.locale to return something meaningful when there are no locales.

lib/ui/platform_dispatcher.dart

@@ -549,9 +549,10 @@ class PlatformDispatcher {
   /// This is the first locale selected by the user and is the user's primary
   /// locale (the locale the device UI is displayed in)
   ///
-  /// 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 => locales.first;
+  /// This is equivalent to `locales.first`, except that it will provide an
+  /// undefined (using the language tag "und") non-null locale if the [locales]
+  /// list has not been set or is empty.
+  Locale get locale => locales.isEmpty ? const Locale.fromSubtags() : locales.first;
 
   /// The full system-reported supported locales of the device.
   ///
@@ -1318,6 +1319,10 @@ class Locale {
   /// [region](https://github.com/unicode-org/cldr/blob/master/common/validity/region.xml) for
   /// each of languageCode, scriptCode and countryCode respectively.
   ///
+  /// The [languageCode] subtag is optional. When there is no language subtag,
+  /// the parameter should be omitted or set to "und". When not supplied, the
+  /// [languageCode] defaults to "und", an undefined language code.
+  ///
   /// The [countryCode] subtag is optional. When there is no country subtag,
   /// the parameter should be omitted or passed `null` instead of an empty-string.
   ///

lib/web_ui/lib/src/engine/platform_dispatcher.dart

@@ -546,6 +546,10 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
     EngineSemanticsOwner.instance.updateSemantics(update);
   }
 
+  /// This is equivalent to `locales.first`, except that it will provide an
+  /// undefined (using the language tag "und") non-null locale if the [locales]
+  /// list has not been set or is empty.
+  ///
   /// We use the first locale in the [locales] list instead of the browser's
   /// built-in `navigator.language` because browsers do not agree on the
   /// implementation.
@@ -554,7 +558,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
   ///
   /// * https://developer.mozilla.org/en-US/docs/Web/API/NavigatorLanguage/languages,
   ///   which explains browser quirks in the implementation notes.
-  ui.Locale get locale => locales.first;
+  ui.Locale get locale => locales.isEmpty ? const ui.Locale.fromSubtags() : locales.first;
 
   /// The full system-reported supported locales of the device.
   ///

lib/web_ui/test/engine/window_test.dart

@@ -292,6 +292,7 @@ void testMain() {
     // that the list is populated again).
     EnginePlatformDispatcher.instance.debugResetLocales();
     expect(window.locales, isEmpty);
+    expect(window.locale, equals(const ui.Locale.fromSubtags()));
     expect(localeChangedCount, 0);
     html.window.dispatchEvent(html.Event('languagechange'));
     expect(window.locales, isNotEmpty);

testing/dart/window_hooks_integration_test.dart

@@ -426,4 +426,17 @@ void main() {
     expectEquals(window.padding.bottom, 0.0);
     expectEquals(window.systemGestureInsets.bottom, 44.0);
   });
+
+  test('PlatformDispatcher.locale returns unknown locale when locales is set to empty list', () {
+    late Locale locale;
+    runZoned(() {
+      window.onLocaleChanged = () {
+        locale = PlatformDispatcher.instance.locale;
+      };
+    });
+
+    _updateLocales(<String>[]);
+    expectEquals(locale, const Locale.fromSubtags());
+    expectEquals(locale.languageCode, 'und');
+  });
 }