[dart:_http] Fix http_cookie_date_test.dart compile errors.
[dart:_http] Fix http_cookie_date_test.dart compile errors.

Fixes https://github.com/dart-lang/sdk/issues/28843
Change-Id: I544f9d13e68a41412b0a7dd00b7a6d20ce3ef99e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/91222
Commit-Queue: Jonas Termansen <092ec58085ae5e368bc39d18752740ff81a2d09c@google.com>
Reviewed-by: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@google.com>

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