Prepare to publish stable null safety (#178)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8084be6..f8fe237 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,6 @@
-## 1.15.0-nullsafety.6-dev
+## 1.15.0
+
+* Stable release for null safety.
 
 ## 1.15.0-nullsafety.5
 
diff --git a/pubspec.yaml b/pubspec.yaml
index cb0b165..03a3f54 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: collection
-version: 1.15.0-nullsafety.6-dev
+version: 1.15.0
 
 description: Collections and utilities functions and classes related to collections.
 homepage: https://github.com/dart-lang/collection