Revert "Pass null instead of 'none' locale" (flutter#6622)

* Revert "Revert "Roll buildtools to 759274dd801b2f78ae0c4066101d0beca27ffc9a" (flutter#6621)"

This reverts commit 9de880c.

* Revert "Properly gate android API < 21 and remove extraneous data array. (flutter#6619)"

This reverts commit 8cb3965.

* Revert "Update Dart version in Dockerfile (flutter#6618)"

This reverts commit 2091c0d.

* Revert "Don't attempt to build any platform target on Fuchsia. (flutter#6617)"

This reverts commit 739d66e.

* Revert "Roll Dart to 45f9462 (flutter#6616)"

This reverts commit 7d49036.

* Revert "Pass null instead of 'none' locale (flutter#6613)"

This reverts commit 6c84c5f.

lib/ui/window.dart

@@ -152,6 +152,9 @@ class Locale {
   /// the region subtag should be uppercase.
   const Locale(this._languageCode, [ this._countryCode ]) : assert(_languageCode != null);
 
+  /// Empty locale constant. This is an invalid locale.
+  static const Locale none = const Locale('', '');
+
   /// The primary language subtag for the locale.
   ///
   /// This must not be null.
@@ -440,7 +443,7 @@ class Window {
     if (_locales != null && _locales.isNotEmpty) {
       return _locales.first;
     }
-    return null;
+    return Locale.none;
   }
 
   /// The full system-reported supported locales of the device.