Update CHANGELOG to reflect breaking change to PartOfDirective constructor.
Update CHANGELOG to reflect breaking change to PartOfDirective constructor.

The breaking change was made in b923dafd253657b9ecb38e316ca3f37d0afe7846.

R=brianwilkerson@google.com

Review URL: https://codereview.chromium.org/2528353005 .

dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk