Update SDK constraint for latest package:js

Author: parlough

pubspec.yaml

@@ -5,7 +5,7 @@ description: A non-interactive HTML documentation generator for Dart source code
 repository: https://github.com/dart-lang/dartdoc
 
 environment:
-  sdk: '>=2.17.0 <3.0.0'
+  sdk: '>=2.19.0-0 <3.0.0'
 
 dependencies:
   analyzer: "^5.2.0"