[dart2js] Tiny cleanup
[dart2js] Tiny cleanup

Inside masks.dart using '_closedWorld.abstractValueDomain' is the same as using 'this'

Change-Id: I30384f6f542283bc770bbf2b0bebdbb12c3c0306
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/156800
Commit-Queue: Christian Altamirano <6cd2b476b177a2af9b6bb95dbc1a0e5062dea0d3@google.com>
Commit-Queue: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@google.com>
Reviewed-by: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@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