Remove -dev.infinity in SDK upper constraint

No longer require
diff --git a/pubspec.yaml b/pubspec.yaml
index 4bd9af6..6165c60 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -9,4 +9,4 @@
   This library declares readable symbolic names for character codes.
 homepage: https://github.com/dart-lang/charcode
 environment:
-  sdk: '>=1.0.0 <2.0.0-dev.infinity'
+  sdk: '>=1.0.0 <2.0.0-dev'