[vm/sendandexit] Add CHANGELOG for Isolate.exit([port, message]) change.

This is follow-up to 56403a0.

Change-Id: I222c04fd4d41e272eeb3e8fd62c55035eee1c12c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/215143
Reviewed-by: Kevin Moore <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

CHANGELOG.md

@@ -944,6 +944,9 @@ This is a patch release that fixes:
 - Add `debugName` positional parameter to `ReceivePort` and `RawReceivePort`
   constructors, a name which can be associated with the port and displayed in
   tooling.
+- Introduce `Isolate.exit([port, message])` which terminates current isolate
+  and, if `port` is specified, as a last action sends out the `message` out to
+  that `port`.
 
 #### `dart:html`