Support browser caching using hashes in serverMode (fixes #93, fixes #92)

test/codegen/expect/server_mode/dev_compiler/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; };' +
         // TODO(jmesserly): arrow functions on V8 don't have lexical this yet.
         // https://code.google.com/p/v8/issues/detail?id=2700
         '["foo"]() { var self = this; return x => self.x + x; };' +
       '};' +
       'return new C(42).foo()(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"';
   console.error(message);
   alert(message);
 
 })();