fix bot: add missing definition in isolate lib for dartc.

lib/isolate/isolate_compiler.dart

 // Copyright (c) 2011, 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.
 
 
 // Dart isolate's API for the dartc compiler (only used for type checking in the
 // dart editor, but not for code generation).
 #library("dart:isolate");
 
 #source("isolate_api.dart");
 
 SendPort _spawnFunction(void topLevelFunction()) {
   throw new NotImplementedException();
 }
 
 SendPort _spawnUri(String uri) {
   throw new NotImplementedException();
 }
 
+ReceivePort _port = null;
+
 class _IsolateNatives {
   static Future<SendPort> spawn(Isolate isolate, bool isLight) {
     throw new NotImplementedException();
   }
 }
 
 class _ReceivePortFactory {
 
   factory ReceivePort() {
     throw new NotImplementedException();
   }
 
   factory ReceivePort.singleShot() {
     throw new NotImplementedException();
   }
 }