VM library tweaks to avoid megamorphic calls in dart2js

runtime/lib/bool_patch.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.
 
 // Dart core library.
 
 patch class bool {
 
   /* patch */ const factory bool.fromEnvironment(String name,
                                                  {bool defaultValue: false})
       native "Bool_fromEnvironment";
 
   int get _identityHashCode {
     return this ? 1231 : 1237;
   }
+  int get hashCode => _identityHashCode;
 }