Use standard harmony flag

lib/runtime/harmony_feature_check.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.
 
 // Feature test for harmony support, alert if not present.
 (function () {
   try {
     var f = new Function(
       '"use strict";'+
       'class C {' +
         'constructor(x) { this.x = x; };' +
         '["foo"]() { return x => this.x + x; };' +
         'bar(args) { return this.foo()(...args); };' +
       '};' +
       'return new C(42).bar([100]);');
     if (f() == 142) return; // supported!
   } catch (e) {
   }
 
   var message = 'This script needs EcmaScript 6 features ' +
       'like `class` and `=>`. Please run in a browser with support, ' +
-      'for example: chrome --js-flags="--harmony-arrow-functions' +
-      ' --harmony-classes --harmony-computed-property-names' +
-      ' --harmony-spreadcalls"';
+      'for example: chrome --js-flags="--harmony"';
   console.error(message);
   alert(message);
 
 })();