Fix some corelib tests.

tests/corelib/iterable_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.
 
 // Regression test for dart2js where [List.addAll] was not typed
 // correctly.
 
 import "package:expect/expect.dart";
 
 import 'dart:collection';
 
-class MyIterable implements IterableBase {
+class MyIterable extends IterableBase {
   get iterator => [].iterator;
 }
 
 main() {
   Expect.isTrue(([]..addAll(new MyIterable())).isEmpty);
 }