fix unsound cast failures in tests

tests/lib_strong/collection/linked_list_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 'dart:collection';
 import "package:expect/expect.dart";
 
 class MyEntry extends LinkedListEntry<MyEntry> {
   final int value;
 
@@ skipping 106 matching lines @@
   var list1 = new LinkedList<MyEntry>();
   list1.addFirst(new MyEntry(0));
 
   var list2 = new LinkedList<MyEntry>();
   Expect.throws(() => list2.addFirst(list1.first));
 
   Expect.throws(() => new MyEntry(0).unlink());
 }
 
 testConcurrentModificationError() {
-  test(function(LinkedList ll)) {
+  test(function(LinkedList<MyEntry> ll)) {
     var ll = new LinkedList<MyEntry>();
     for (int i = 0; i < 10; i++) {
       ll.add(new MyEntry(i));
     }
     Expect.throws(() => function(ll), (e) => e is ConcurrentModificationError);
   }
 
   test((ll) {
     for (var x in ll) {
       ll.remove(x);
@@ skipping 81 matching lines @@
     });
   });
 }
 
 main() {
   testInsert();
   testRemove();
   testBadAdd();
   testConcurrentModificationError();
 }