Implement modular compilation

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"';
-  console.error(message);
-  alert(message);
-
-})();