Add library doc comment to dart:cli
Add library doc comment to dart:cli

Needed to fix broken build:
https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8832225116390642513/+/u/build_api_docs/stdout

Cq-Include-Trybots: luci.dart.try:dart-sdk-linux-try
Change-Id: I9d1b5a94781de4a4380aebf5ccbabbb0d31a75d6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/218420
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>
Reviewed-by: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>
Commit-Queue: Michael Thomsen <5977a55c75bb4612004451a56b0b4c22d9d3311d@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk