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

I anticipate heavily modifying this file, and I don't want to have to
mix comment styles when I do so.

Change-Id: Ib93bf5489f080456da81f03f82bd7e850498748c
Reviewed-on: https://dart-review.googlesource.com/73683
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>
Commit-Queue: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>

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