Support user generated custom native JS classes.

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 363 matching lines @@
         split(pattern) => [];
         int get length => 42;
         operator[](index) {}
         toString() {}
         operator+(other) => this;
         codeUnitAt(index) => 42;
       }''',
   'JSUInt31': 'class JSUInt31 extends JSUInt32 {}',
   'JSUInt32': 'class JSUInt32 extends JSPositiveInt {}',
   'ObjectInterceptor': 'class ObjectInterceptor {}',
+  'JavaScriptObject': 'class JavaScriptObject {}',
   'PlainJavaScriptObject': 'class PlainJavaScriptObject {}',
   'UnknownJavaScriptObject': 'class UnknownJavaScriptObject {}',
+  'JavaScriptFunction': 'class JavaScriptFunction {}',
 };
 
 const Map<String, String> DEFAULT_ISOLATE_HELPER_LIBRARY =
     const <String, String>{
   'startRootIsolate': 'void startRootIsolate(entry, args) {}',
   '_currentIsolate': 'var _currentIsolate;',
   '_callInIsolate': 'var _callInIsolate;',
   '_WorkerBase': 'class _WorkerBase {}',
 };
 
@@ skipping 49 matching lines @@
 
     const LookupMap(this._entries, [this._nestedMaps = const []])
         : _key = null, _value = null;
 
     const LookupMap.pair(this._key, this._value)
         : _entries = const [], _nestedMaps = const [];
     V operator[](K k) => null;
   }''',
   '_version': 'const _version = "0.0.1+1";',
 };