Remove -dev.infinity in SDK upper constraint

No longer require
diff --git a/pubspec.yaml b/pubspec.yaml
index 9f4c1c5..41dd9e9 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -5,7 +5,7 @@
 homepage: https://github.com/dart-lang/convert
 
 environment:
-  sdk: '>=1.17.0 <2.0.0-dev.infinity'
+  sdk: '>=1.17.0 <2.0.0'
 
 dependencies:
   charcode: '^1.1.0'