Migrated test block 16 to Dart 2.0.

tests/corelib_strong/map_index_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();
-}
-
-main() {
-  var map1 = new Map<B, B>();
-  map1[const B()] = const B();
-  var map2 = new Map<B, B>();
-  var list = <B>[const B()];
-
-  var maps = [map1, map2];
-  for (var map in maps) {
-    // Test that the map accepts a key is not of the same type:
-    //   Map<B, ?>[A]
-    Expect.isNull(map[new A()]);
-  }
-}