Make a new version of package:unmodifiable_collection that defers to collection_helpers.

pkg/unmodifiable_collection/lib/unmodifiable_collection.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/**
+ * This library used to introduce unmodifiable wrappers for collections.
+ *
+ * This functionality has been moved to the `collection_helpers` library.
+ *
+ * Please replace the import of this library with:
+ *
+ *     import "package:collection_helpers/wrappers.dart";
+ *
+ * and change dependencies to match.
+ */
+@deprecated
+library unmodifiable_collection;
+
+export "package:collection_helpers/wrappers.dart"
+    show UnmodifiableListView,
+         UnmodifiableSetView,
+         UnmodifiableMapView,
+         NonGrowableListView;