Clarify behavior of sublist with end == start
Clarify behavior of sublist with end == start

Closes #28074

I used some wording from the discussion. I kept he title line as is
since I think the "inclusive/exclusive" distinction is how most readers
think of it, and the more precise language can be kept in the details.

Change-Id: I4de2ffc77dbc8168efcacb94a168c1a2428a5293
Reviewed-on: https://dart-review.googlesource.com/71390
Reviewed-by: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>
Commit-Queue: Nate Bosch <dc212c74d8afde9c690fdd09e74d3e5c5d5a3de0@google.com>

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