[fixnum] update the min. required dart sdk (#907)

pkgs/fixnum/CHANGELOG.md

diff --git a/pkgs/fixnum/CHANGELOG.md b/pkgs/fixnum/CHANGELOG.md
index a505c3e..a378853 100644
--- a/pkgs/fixnum/CHANGELOG.md
+++ b/pkgs/fixnum/CHANGELOG.md
@@ -1,9 +1,10 @@
 ## 1.2.0-wip
 
-- Change `IntX` such that it implements `Comparable<IntX>`. This makes it
+* Change `IntX` such that it implements `Comparable<IntX>`. This makes it
   possible for `IntX` (and in turn `Int64` and `Int32`) be used with methods
   like `sortedBy` in the platform libraries.
-- Run `dart format` with the new style.
+* Run `dart format` with the new style.
+* Require Dart 3.4
 
 ## 1.1.1
 

pkgs/fixnum/pubspec.yaml

diff --git a/pkgs/fixnum/pubspec.yaml b/pkgs/fixnum/pubspec.yaml
index fb9c546..4fb2e40 100644
--- a/pkgs/fixnum/pubspec.yaml
+++ b/pkgs/fixnum/pubspec.yaml
@@ -7,7 +7,7 @@
 issue_tracker: https://github.com/dart-lang/core/issues?q=is%3Aissue+is%3Aopen+label%3Apackage%3Afixnum
 
 environment:
-  sdk: ^3.1.0
+  sdk: ^3.4.0
 
 dev_dependencies:
   dart_flutter_team_lints: ^3.0.0