Prepare to publish (#214)

Drop `-dev` postfix.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2326344..b5b5c9e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,4 @@
-## 2.9.0-dev
+## 2.9.0
 
 * **Potentially Breaking** The default `propagateCancel` argument to
   `CancelableOperation.then` changed from `false` to `true`. In most usages this
diff --git a/pubspec.yaml b/pubspec.yaml
index 10d7d8f..2275bce 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: async
-version: 2.9.0-dev
+version: 2.9.0
 
 description: Utility functions and classes related to the 'dart:async' library.
 repository: https://github.com/dart-lang/async