Add 2.15 release date

Fixes https://github.com/dart-lang/sdk/issues/47909

Bug: https://github.com/dart-lang/sdk/issues/47909
Change-Id: I7a2c81ba3888b52db2f69df36e17c58de7522179
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/223424
Reviewed-by: Alexander Thomas <athom@google.com>
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 185379d..2436af7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -64,7 +64,7 @@
 - adds new lint: `use_decorated_box`.
 - improves docs for `omit_local_variable_types`.
 
-## 2.15.0
+## 2.15.0 - 2021-12-08
 
 ### Language