Change date format in CHANGELOG.md to yyyy-mm-dd uniformly.
Change date format in CHANGELOG.md to yyyy-mm-dd uniformly.

There were previously both dd-mm-yyyy and yyyy-mm-dd.

Change-Id: I3eb49c61bc032ff4986283bcc78e4410c0d007ac
Reviewed-on: https://dart-review.googlesource.com/69309
Reviewed-by: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@google.com>

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