Allow Object when doing lookups.

tests/corelib/iterable_contains2_test.dart

+// Copyright (c) 2012, 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";
+
+// Tests for the contains methods on lists.
+
+class A { const A(); }
+class B extends A { const B(); }
+
+main() {
+  var list = <B>[ new B() ];
+  var set = new Set<B>();
+  set.add(new B());
+  var iterable1 = list.map((x) => x);
+  var iterable2 = list.take(1);
+  var list2 = const <B>[ const B() ];
+  var iterable3 = list2.map((x) => x);
+  var iterable4 = list2.take(1);
+  var iterables =
+      [ list, set, iterable1, iterable2, list2, iterable3, iterable4 ];
+  for (var iterable in iterables) {
+    Expect.isFalse(iterable.contains(new A()));
+  }
+}