Combine unmodifiable_collection package into collection_helpers.

pkg/collection_helpers/test/unmodifiable_collection_test.dart

Index: pkg/collection_helpers/test/unmodifiable_collection_test.dart
diff --git a/pkg/unmodifiable_collection/test/unmodifiable_collection_test.dart b/pkg/collection_helpers/test/unmodifiable_collection_test.dart
similarity index 99%
rename from pkg/unmodifiable_collection/test/unmodifiable_collection_test.dart
rename to pkg/collection_helpers/test/unmodifiable_collection_test.dart
index 7e6d3fc218017b92da70b42da242ac236eb39bab..ac5bb079201c6d49a995545f6fc7644ceca538da 100644
--- a/pkg/unmodifiable_collection/test/unmodifiable_collection_test.dart
+++ b/pkg/collection_helpers/test/unmodifiable_collection_test.dart
@@ -2,7 +2,7 @@
 // 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.
 
-import "package:unmodifiable_collection/unmodifiable_collection.dart";
+import "package:collection_helpers/wrappers.dart";
 import "package:unittest/unittest.dart";
 
 // Test unmodifiable collection views.