Add trailing comma support to changelog

BUG=
R=lrn@google.com

Review URL: https://codereview.chromium.org/2220963002 .

CHANGELOG.md

@@ -1,5 +1,13 @@
 ## 1.19.0
 
+### Language changes
+
+* The language now allows a final argument or parameter in an argument or
+ parameter list to have an optional trailing comma. This can make long argument
+ or parameter lists easier to maintain, as commas can be left as-is when
+ reordering lines. For details, see SDK issue
+ [26644](https://github.com/dart-lang/sdk/issues/26644).
+
 ### Core library changes
 
 * `dart:io`