Enable ES6 unscopables

test/mjsunit/es6/unscopables.js

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --harmony-unscopables
 // Flags: --harmony-collections
 
 var global = this;
 var globalProto = Object.getPrototypeOf(global);
 
@@ skipping 644 matching lines @@
       throw new CustomError();
     }
   });
   assertThrows(function() {
     with (object) {
       x;
     }
   }, CustomError);
 }
 TestGetUnscopablesGetterThrows();