anchor.diff:      });
       group("trackBy function by index", () {
         var differ;
  -      var trackByIndex = (num index, dynamic item) => index;
  +      var trackByIndex = (int index, Object item) => index;
         setUp(() {
           differ = new DefaultIterableDiffer(trackByIndex);
         });
  
positive.code: import 'dart:collection';
  
  import 'package:test/test.dart';
  import 'package:_tests/test_util.dart';
  import 'package:angular/src/core/change_detection/differs/default_iterable_differ.dart';
  
  void main() {
  
negative.code:             differ.toString(),
              iterableChangesAsString(
                  collection: ["a", "boo"], previous: ["a", "boo"]));
        });
        test("should remove and add same item", () {
          var l = ["a", "b", "c"];
  