Migrate test block 3 to Dart 2.0.

tests/corelib_2/bool_hashcode_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";
 
 class BoolHashCodeTest {
   static testMain() {
     Expect.notEquals(true.hashCode, false.hashCode);
   }
 }
 
 main() {
   BoolHashCodeTest.testMain();
 }