Add changelog entry for RegExp lookbehind support.

Change-Id: Iafc99d75bcfeaa5a236bdff7808bace1b153445e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/96945
Auto-Submit: Stevie Strickland <sstrickl@google.com>
Reviewed-by: Kevin Moore <kevmoo@google.com>
Commit-Queue: Kevin Moore <kevmoo@google.com>
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0f2e68e..266cce2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,8 @@
 
 ### Dart VM
 
+* RegExp patterns can now use lookbehind assertions.
+
 ### Tool Changes
 
 #### Pub client