Migrated test block 25 to Dart 2.0.

tests/corelib_2/set_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 set1 = new Set<B>();
   set1.add(const B());
   var set2 = new Set<B>();
   var list = <B>[const B()];
   var set3 = list.toSet();
 
   var sets = [set1, set2, set3];
   for (var setToTest in sets) {
     // Test that the set accepts a value that is not of the same type
     //   Set<B>.remove(A)
     Expect.isFalse(setToTest.remove(new A()));
   }
 }