Fix new unused import diagnostic (#198)
The check is now stronger and surfaces imports that are only exposing
symbols that are also available through other imports.
diff --git a/lib/src/empty_unmodifiable_set.dart b/lib/src/empty_unmodifiable_set.dart
index 44bd5ef..882b6c8 100644
--- a/lib/src/empty_unmodifiable_set.dart
+++ b/lib/src/empty_unmodifiable_set.dart
@@ -6,8 +6,6 @@
import 'package:collection/collection.dart';
-import 'unmodifiable_wrappers.dart';
-
/// An unmodifiable, empty set which can be constant.
class EmptyUnmodifiableSet<E> extends IterableBase<E>
with UnmodifiableSetMixin<E>