implement null aware ops, fixes #249

lib/runtime/dart_runtime.js

 // Copyright (c) 2015, 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_library.library('dart_runtime/dart', null, /* Imports */[
   'dart_runtime/_classes',
   'dart_runtime/_errors',
   'dart_runtime/_generators',
   'dart_runtime/_operations',
   'dart_runtime/_rtti',
@@ skipping 69 matching lines @@
     'dindex',
     'dload',
     'dput',
     'dsend',
     'dsetindex',
     'equals',
     'hashCode',
     'map',
     'noSuchMethod',
     'notNull',
+    'nullSafe',
     'stackPrint',
     'stackTrace',
     'strongInstanceOf',
     'throw',
     'toString',
   ])
   // Renames
   exports.as = _export(operations.cast);
   exports.is = _export(operations.instanceOf);
 
@@ skipping 10 matching lines @@
   ]);
 
   // From rtti
   exportFrom(rtti, [
     'fn',
     'realRuntimeType',
     'runtimeType',
   ]);
 
 });