prep package:js for null safety release

Change-Id: Ie4ce3d83d4dfc60ff5728152e9b391a84c50e6b8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153824
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Auto-Submit: Jake Macdonald <jakemac@google.com>

Author: jakemac53

pkg/js/CHANGELOG.md

@@ -1,4 +1,4 @@
-## 0.6.3-dev
+## 0.6.3-nullsafety
 
 * Opt in to null safety.
 

pkg/js/pubspec.yaml

@@ -1,10 +1,10 @@
 name: js
-version: 0.6.3-dev
+version: 0.6.3-nullsafety
 description: Annotations to create static Dart interfaces for JavaScript APIs.
 homepage: https://github.com/dart-lang/sdk/tree/master/pkg/js
 
 environment:
-  sdk: '>=2.9.0-1 <3.0.0'
+  sdk: '>=2.9.0-18.0 <2.9.0'
 
 dev_dependencies:
   pedantic: ^1.9.0