CHANGELOG for cast/retype changes

Change-Id: I8d23c6829937de3c1d6c42d7e33b106473f82d1d
Reviewed-on: https://dart-review.googlesource.com/54522
Reviewed-by: Bob Nystrom <rnystrom@google.com>
diff --git a/CHANGELOG.md b/CHANGELOG.md
index d347192..16ddcd7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,14 @@
 
 ### Language
 
+* Changed the `cast` method to always change the type.  Deprecated the
+  `retype` method and made it redirect to `cast`.  Applies to all of the
+  following interfaces:
+  * `Stream`:
+  * `StreamTransformer`
+  * `Iterable`
+  * `Map`
+
 #### Strong Mode
 
 ### Core library changes