Allow Object when doing lookups.

tests/corelib/list_index_of2_test.dart

+// Copyright (c) 2011, 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:expect/expect.dart";
+
+class A { const A(); }
+class B extends A { const B(); }
+
+void test(List<B> list) {
+  // Test that the list accepts a different type for indexOf.
+  //   List<B>.indexOf(A)
+  Expect.equals(-1, list.indexOf(const A()));
+  Expect.equals(-1, list.lastIndexOf(const A()));
+}
+
+main() {
+  var list = new List<B>(1);
+  list[0] = const B();
+  test(list);
+  var list2 = new List<B>();
+  list2.add(const B());
+  test(list2);
+  test(<B>[const B()]);
+  test(const <B>[]);
+  test(<B>[const B()].toList());
+}