Convert link.dart to triple-slash comment style.
Convert link.dart to triple-slash comment style.

I'm going to be adding a lot of new functionality to this file, and I
don't want to have to mix comment styles when I do so.

Change-Id: I35fb7b93b2cfc200995364028ea10bab9c8f1245
Reviewed-on: https://dart-review.googlesource.com/73640
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>
Reviewed-by: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>
Commit-Queue: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>

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