update SDK constraint for null safety
diff --git a/pubspec.yaml b/pubspec.yaml
index f997bdb..7c31136 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -5,4 +5,4 @@
 homepage: https://github.com/dart-lang/pedantic
 
 environment:
-  sdk: '>=2.8.0-dev <2.8.0'
+  sdk: '>=2.9.0-1 <2.9.0'