Migrated test block 16 to Dart 2.0.

tests/corelib_2/map_remove_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, ?>.remove(A)
     Expect.isNull(map.remove(new A()));
   }
 }