diff --git a/lib/date_time_patterns.dart b/lib/date_time_patterns.dart
index ec18613..dfce6cf 100644
--- a/lib/date_time_patterns.dart
+++ b/lib/date_time_patterns.dart
@@ -1,7 +1,6 @@
 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
-// @dart=2.9
 
 /// Date/time formatting symbols for a large subset of locales.
 ///
diff --git a/lib/number_symbols.dart b/lib/number_symbols.dart
index 8441bd0..2f5c4bf 100644
--- a/lib/number_symbols.dart
+++ b/lib/number_symbols.dart
@@ -1,7 +1,6 @@
 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
-// @dart=2.9
 library number_symbols;
 
 // Suppress naming issues as changes would be breaking.
@@ -30,22 +29,22 @@
       DEF_CURRENCY_CODE;
 
   const NumberSymbols(
-      {this.NAME,
-      this.DECIMAL_SEP,
-      this.GROUP_SEP,
-      this.PERCENT,
-      this.ZERO_DIGIT,
-      this.PLUS_SIGN,
-      this.MINUS_SIGN,
-      this.EXP_SYMBOL,
-      this.PERMILL,
-      this.INFINITY,
-      this.NAN,
-      this.DECIMAL_PATTERN,
-      this.SCIENTIFIC_PATTERN,
-      this.PERCENT_PATTERN,
-      this.CURRENCY_PATTERN,
-      this.DEF_CURRENCY_CODE});
+      {required this.NAME,
+      required this.DECIMAL_SEP,
+      required this.GROUP_SEP,
+      required this.PERCENT,
+      required this.ZERO_DIGIT,
+      required this.PLUS_SIGN,
+      required this.MINUS_SIGN,
+      required this.EXP_SYMBOL,
+      required this.PERMILL,
+      required this.INFINITY,
+      required this.NAN,
+      required this.DECIMAL_PATTERN,
+      required this.SCIENTIFIC_PATTERN,
+      required this.PERCENT_PATTERN,
+      required this.CURRENCY_PATTERN,
+      required this.DEF_CURRENCY_CODE});
 
   String toString() => NAME;
 }
@@ -56,10 +55,10 @@
 /// number formatting pattern.)
 class CompactNumberSymbols {
   final Map<int, String> COMPACT_DECIMAL_SHORT_PATTERN;
-  final Map<int, String> COMPACT_DECIMAL_LONG_PATTERN;
+  final Map<int, String>? COMPACT_DECIMAL_LONG_PATTERN;
   final Map<int, String> COMPACT_DECIMAL_SHORT_CURRENCY_PATTERN;
   CompactNumberSymbols(
-      {this.COMPACT_DECIMAL_SHORT_PATTERN,
+      {required this.COMPACT_DECIMAL_SHORT_PATTERN,
       this.COMPACT_DECIMAL_LONG_PATTERN,
-      this.COMPACT_DECIMAL_SHORT_CURRENCY_PATTERN});
+      required this.COMPACT_DECIMAL_SHORT_CURRENCY_PATTERN});
 }
diff --git a/lib/number_symbols_data.dart b/lib/number_symbols_data.dart
index 11d60eb..8536d1c 100644
--- a/lib/number_symbols_data.dart
+++ b/lib/number_symbols_data.dart
@@ -3,7 +3,6 @@
 // for details. All rights reserved. Use of this source
 // code is governed by a
 // BSD-style license that can be found in the LICENSE file.
-// @dart=2.9
 
 /// Date/time formatting symbols for all locales.
 ///
diff --git a/lib/src/data/dates/locale_list.dart b/lib/src/data/dates/locale_list.dart
index 40bdc31..1581cca 100644
--- a/lib/src/data/dates/locale_list.dart
+++ b/lib/src/data/dates/locale_list.dart
@@ -1,7 +1,6 @@
 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this sourcecode is governed by a
 // BSD-style license that can be found in the LICENSE file.
-// @dart=2.9
 
 /// Hard-coded list of all available locales for dates.
 final availableLocalesForDateFormatting = const [
diff --git a/lib/src/locale/locale_deprecations.dart b/lib/src/locale/locale_deprecations.dart
index 31adf3b..f43b6b7 100644
--- a/lib/src/locale/locale_deprecations.dart
+++ b/lib/src/locale/locale_deprecations.dart
@@ -1,7 +1,6 @@
 // Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
-// @dart=2.9
 
 /// Replaces deprecated language subtags.
 ///
@@ -102,7 +101,7 @@
 /// The subtag must already be uppercase.
 ///
 /// TODO(b/127689510): write a new script for updating this list from CLDR data.
-String replaceDeprecatedRegionSubtag(String regionCode) {
+String? replaceDeprecatedRegionSubtag(String? regionCode) {
   return _deprecatedRegionTagReplacements[regionCode] ?? regionCode;
 }
 
diff --git a/lib/src/locale/locale_extensions.dart b/lib/src/locale/locale_extensions.dart
index e7ff25a..c04937d 100644
--- a/lib/src/locale/locale_extensions.dart
+++ b/lib/src/locale/locale_extensions.dart
@@ -1,7 +1,6 @@
 // Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
-// @dart=2.9
 
 /// Locale extensions as defined for [Unicode Locale
 /// Identifiers](http://www.unicode.org/reports/tr35/#Unicode_locale_identifier).
@@ -15,9 +14,9 @@
   /// Keys in each of the maps passed to this contructor must be syntactically
   /// valid extension keys, and must already be normalized (correct case).
   LocaleExtensions(
-      Map<String, String> uExtensions,
-      Map<String, String> tExtensions,
-      Map<String, String> otherExtensions,
+      Map<String, String>? uExtensions,
+      Map<String, String>? tExtensions,
+      Map<String, String>? otherExtensions,
       this._xExtensions)
       : _uExtensions = _sortedUnmodifiable(uExtensions),
         _tExtensions = _sortedUnmodifiable(tExtensions),
@@ -71,7 +70,7 @@
         'RegExp/${_otherExtensionsValidValuesRE.pattern}. '
         'Entries: ${otherExtensions.entries}.');
     assert(
-        _xExtensions == null || _validXExtensionsRE.hasMatch(_xExtensions),
+        _xExtensions == null || _validXExtensionsRE.hasMatch(_xExtensions!),
         '_xExtensions must match RegExp/${_validXExtensionsRE.pattern}/ '
         'but is "$_xExtensions".');
   }
@@ -165,7 +164,7 @@
 
   /// -x- extension values. See
   /// http://www.unicode.org/reports/tr35/#pu_extensions for details.
-  final String _xExtensions;
+  final String? _xExtensions;
 
   /// List of subtags in the [Unicode Locale
   /// Identifier](https://www.unicode.org/reports/tr35/#Unicode_locale_identifier)
@@ -218,13 +217,13 @@
 }
 
 /// Creates an unmodifiable and sorted version of `unsorted`.
-Map<String, String> _sortedUnmodifiable(Map<String, String> unsorted) {
+Map<String, String> _sortedUnmodifiable(Map<String, String>? unsorted) {
   if (unsorted == null) {
     return const {};
   }
   var map = <String, String>{};
   for (var key in unsorted.keys.toList()..sort()) {
-    map[key] = unsorted[key];
+    map[key] = unsorted[key]!;
   }
   return Map.unmodifiable(map);
 }