Combine unmodifiable_collection package into collection_helpers.

pkg/collection_helpers/test/unmodifiable_collection_test.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.
 
-import "package:unmodifiable_collection/unmodifiable_collection.dart";
+import "package:collection_helpers/wrappers.dart";
 import "package:unittest/unittest.dart";
 
 // Test unmodifiable collection views.
 // The collections should pass through the operations that are allowed,
 // an throw on the ones that aren't without affecting the original.
 
 main() {
   List list = [];
   testUnmodifiableList(list, new UnmodifiableListView(list), "empty");
   list = [42];
@@ skipping 604 matching lines @@
  });
 
  testThrows("$name remove throws", () {
    wrapped.remove(0);
  });
 
  testThrows("$name clear throws", () {
    wrapped.clear();
  });
 }