[vm] Fix comment compact hash implementation
[vm] Fix comment compact hash implementation

The `_CompactIterableImmutable` does not check for concurrent
modifications, because the underlying map is immutable.

The comment was wrong.

Change-Id: I992d841cb6b42d027e8ae5bb853c8345a0f7a69d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/245160
Commit-Queue: Daco Harkes <930cbae0ca77f1954c53e32620d89cfcca318f00@google.com>
Reviewed-by: Lasse Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@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