Redo "Constant fold charCodeAt via constant system"

tests/compiler/dart2js/mock_libraries.dart

 // Copyright (c) 2014, 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.
 
 // Library for creating mock versions of platform and internal libraries.
 
 library mock_libraries;
 
 String buildLibrarySource(
     Map<String, String> elementMap,
@@ skipping 321 matching lines @@
         remainder(other) => (this is JSInt) ? 42 : 42.2;
         truncate() => 42;
       }''',
   'JSString': r'''
       class JSString extends Interceptor implements String, JSIndexable {
         var split;
         var length;
         operator[](index) {}
         toString() {}
         operator+(other) => this;
+        codeUnitAt(index) => 42;
       }''',
   'JSUInt31': 'class JSUInt31 extends JSUInt32 {}',
   'JSUInt32': 'class JSUInt32 extends JSPositiveInt {}',
   'mapTypeToInterceptor': 'var mapTypeToInterceptor;',
   'ObjectInterceptor': 'class ObjectInterceptor {}',
   'PlainJavaScriptObject': 'class PlainJavaScriptObject {}',
   'UnknownJavaScriptObject': 'class UnknownJavaScriptObject {}',
 };
 
 const Map<String, String> DEFAULT_ISOLATE_HELPER_LIBRARY =
     const <String, String>{
   'startRootIsolate': 'var startRootIsolate;',
   '_currentIsolate': 'var _currentIsolate;',
   '_callInIsolate': 'var _callInIsolate;',
   '_WorkerBase': 'class _WorkerBase {}',
 };
 
 const Map<String, String> DEFAULT_MIRRORS_LIBRARY = const <String, String>{
   'Comment': 'class Comment {}',
   'MirrorSystem': 'class MirrorSystem {}',
   'MirrorsUsed': 'class MirrorsUsed {}',
 };
-