Implementing Locale.toLanguageTag in flutter web. (#17131)

lib/web_ui/lib/src/ui/window.dart

@@ -478,19 +478,17 @@ class Locale {
   int get hashCode => hashValues(languageCode, scriptCode, countryCode);
 
   @override
-  String toString() {
+  String toString() => _rawToString('_');
+
+  // TODO(yjbanov): implement to match flutter native.
+  String toLanguageTag() => _rawToString('-');
+
+  String _rawToString(String separator) {
     final StringBuffer out = StringBuffer(languageCode);
-    if (scriptCode != null) {
-      out.write('_$scriptCode');
-    }
-    if (_countryCode != null) {
-      out.write('_$countryCode');
-    }
+    if (scriptCode != null) out.write('$separator$scriptCode');
+    if (_countryCode != null) out.write('$separator$countryCode');
     return out.toString();
   }
-
-  // TODO(yjbanov): implement to match flutter native.
-  String toLanguageTag() => '_';
 }
 
 /// The most basic interface to the host operating system's user interface.