Add class modifiers (#1034)

diff --git a/pkgs/intl4x/CHANGELOG.md b/pkgs/intl4x/CHANGELOG.md
index 4a88f23..36cb2f1 100644
--- a/pkgs/intl4x/CHANGELOG.md
+++ b/pkgs/intl4x/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 0.16.0
+
+- Add class modifiers.
+
 ## 0.15.0
 
 - API overhaul:
diff --git a/pkgs/intl4x/lib/src/case_mapping/case_mapping.dart b/pkgs/intl4x/lib/src/case_mapping/case_mapping.dart
index c52a301..050b854 100644
--- a/pkgs/intl4x/lib/src/case_mapping/case_mapping.dart
+++ b/pkgs/intl4x/lib/src/case_mapping/case_mapping.dart
@@ -22,7 +22,7 @@
 /// ```
 ///
 /// Caution: During testing, the input is returned unchanged.
-class CaseMapping {
+final class CaseMapping {
   final CaseMappingImpl _caseMappingImpl;
 
   /// Constructs a [CaseMapping] instance for the given [locale].
diff --git a/pkgs/intl4x/lib/src/collation/collation.dart b/pkgs/intl4x/lib/src/collation/collation.dart
index e5bd178..48f2be6 100644
--- a/pkgs/intl4x/lib/src/collation/collation.dart
+++ b/pkgs/intl4x/lib/src/collation/collation.dart
@@ -9,7 +9,7 @@
 import 'collation_options.dart';
 
 /// Provides locale-sensitive string comparison.
-class Collation {
+final class Collation {
   final CollationImpl _collationImpl;
 
   /// Creates a new locale-sensitive string comparator.
diff --git a/pkgs/intl4x/lib/src/datetime_format/datetime_format.dart b/pkgs/intl4x/lib/src/datetime_format/datetime_format.dart
index e74d2ac..fd0a447 100644
--- a/pkgs/intl4x/lib/src/datetime_format/datetime_format.dart
+++ b/pkgs/intl4x/lib/src/datetime_format/datetime_format.dart
@@ -23,7 +23,7 @@
 ///   // Output: '04:00'
 /// }
 /// ```
-class DateTimeFormat {
+sealed class DateTimeFormat {
   /// Formatting just the day.
   ///
   /// Example:
diff --git a/pkgs/intl4x/lib/src/display_names/display_names.dart b/pkgs/intl4x/lib/src/display_names/display_names.dart
index 69f520a..7809eb3 100644
--- a/pkgs/intl4x/lib/src/display_names/display_names.dart
+++ b/pkgs/intl4x/lib/src/display_names/display_names.dart
@@ -12,7 +12,7 @@
 ///
 /// This class uses the specified locale and options to format codes (like 'en',
 ///  'US') into human-readable strings (like 'English', 'United States').
-class DisplayNames {
+final class DisplayNames {
   final DisplayNamesImpl _impl;
 
   /// Creates a new display name formatter configured for a specific locale.
diff --git a/pkgs/intl4x/lib/src/list_format/list_format.dart b/pkgs/intl4x/lib/src/list_format/list_format.dart
index 7a12956..1e6cbc1 100644
--- a/pkgs/intl4x/lib/src/list_format/list_format.dart
+++ b/pkgs/intl4x/lib/src/list_format/list_format.dart
@@ -12,7 +12,7 @@
 ///
 /// This is typically used for joining items with appropriate conjunctions (like
 /// 'and' or 'or') or punctuation (like commas).
-class ListFormat {
+final class ListFormat {
   final ListFormatImpl _listFormatImpl;
 
   /// Creates a new list formatter.
diff --git a/pkgs/intl4x/lib/src/number_format/number_format.dart b/pkgs/intl4x/lib/src/number_format/number_format.dart
index 7ea073a..c84340e 100644
--- a/pkgs/intl4x/lib/src/number_format/number_format.dart
+++ b/pkgs/intl4x/lib/src/number_format/number_format.dart
@@ -23,7 +23,7 @@
 ///   print(NumberFormat.format(123456.789)); // Prints '123,456.789'
 /// }
 /// ```
-class NumberFormat {
+final class NumberFormat {
   final NumberFormatImpl _impl;
 
   /// Creates a [NumberFormat] instance with custom formatting options.
diff --git a/pkgs/intl4x/lib/src/number_format/number_format_options.dart b/pkgs/intl4x/lib/src/number_format/number_format_options.dart
index b3aa99a..db6d190 100644
--- a/pkgs/intl4x/lib/src/number_format/number_format_options.dart
+++ b/pkgs/intl4x/lib/src/number_format/number_format_options.dart
@@ -9,7 +9,7 @@
 typedef UnitDisplay = Style;
 
 /// Number formatting functionality of the browser.
-class NumberFormatOptions {
+final class NumberFormatOptions {
   final FormatStyle style;
   final String? currency;
 
diff --git a/pkgs/intl4x/lib/src/plural_rules/plural_rules.dart b/pkgs/intl4x/lib/src/plural_rules/plural_rules.dart
index 9fd10d0..c8f7824 100644
--- a/pkgs/intl4x/lib/src/plural_rules/plural_rules.dart
+++ b/pkgs/intl4x/lib/src/plural_rules/plural_rules.dart
@@ -12,7 +12,7 @@
 ///
 /// This can be used to select the correct localized string (e.g., "1 cat" vs.
 /// "2 cat**s**").
-class PluralRules {
+final class PluralRules {
   final PluralRulesImpl _pluralRulesImpl;
 
   /// Creates a new plural rules selector.
diff --git a/pkgs/intl4x/pubspec.yaml b/pkgs/intl4x/pubspec.yaml
index a1e00b5..0b70975 100644
--- a/pkgs/intl4x/pubspec.yaml
+++ b/pkgs/intl4x/pubspec.yaml
@@ -1,7 +1,7 @@
 name: intl4x
 description: >-
   A lightweight modular library for internationalization (i18n) functionality.
-version: 0.15.0
+version: 0.16.0
 repository: https://github.com/dart-lang/i18n/tree/main/pkgs/intl4x
 issue_tracker: https://github.com/dart-lang/i18n/issues?q=is%3Aissue+is%3Aopen+label%3Apackage%3Aintl4x