Migrate test block 8 to Dart 2.0.

tests/corelib_2/hash_map2_test.dart

 // Copyright (c) 2013, 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.
 //
 // VMOptions=
 // VMOptions=--use_internal_hash_map
 
 // Tests of hash map behavior, with focus in iteration and concurrent
 // modification errors.
 
@@ skipping 291 matching lines @@
   testMap(() => new HashMap(), (m) => new HashMap.from(m));
   testMap(() => new LinkedHashMap(), (m) => new LinkedHashMap.from(m));
 }
 
 class BadHashCode {
   static int idCounter = 0;
   final int id;
   BadHashCode() : id = idCounter++;
   int get hashCode => 42;
 }