Update changelog with dart2js --package-root removal

Follow-up to b352256

Change-Id: I47e75fc3f7916cc6ec0464ace1b6df93638dd80b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/141163
Reviewed-by: Sigmund Cherem <[email protected]>

Author: kevmoo

CHANGELOG.md

@@ -333,6 +333,13 @@ In addition, we fixed some inconsistencies between Dart2JS and DDC:
 
 [normalization]: https://github.com/dart-lang/language/blob/master/resources/type-system/normalization.md
 
+
+Other dart2js changes:
+
+* **Breaking Change**: The `--package-root` flag, which was hidden and disabled
+  in Dart 2.0.0, has been completely removed. Passing this flag will now cause
+  `dart2js` to fail.
+
 #### Linter
 
 Updated the Linter to `0.1.114`, which includes: