Support Dart 2.11

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index 9ee28a4..5db173f 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -9,7 +9,7 @@
 publish_to: none
 
 environment:
-  sdk: '>=2.11.0-0 <2.11.0'
+  sdk: '>=2.11.0-0 <2.12.0'
 
 dependencies:
   charcode: ^1.2.0-nullsafety