Make dart2js Isolate.spawn work.

tests/isolate/illegal_msg_test.dart

 // Copyright (c) 2012, 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 illegal_msg_tests;
 import "package:expect/expect.dart";
 import 'dart:isolate';
 import "package:async_helper/async_helper.dart";
 
 funcFoo(x) => x + 2;
 
 echo(sendPort) {
   var port = new ReceivePort();
   sendPort.send(port.sendPort);
   port.listen((msg) {
     sendPort.send("echoing ${msg(1)}}");
   });
 }
 
 main() {
   ReceivePort port = new ReceivePort();
-  Future spawn = Isolate.spawn(echo, port.sendPort);
+  Isolate.spawn(echo, port.sendPort);
   var caught_exception = false;
   var stream = port.asBroadcastStream();
   asyncStart();
   stream.first.then((snd) {
     try {
       snd.send(funcFoo);
     } catch (e) {
       caught_exception = true;
     }
 
     if (caught_exception) {
       port.close();
     } else {
       asyncStart();
       stream.first.then((msg) {
         print("from worker ${msg}");
         asyncEnd();
       });
     }
     Expect.isTrue(caught_exception);
     asyncEnd();
   });
 }