Support web workers in dart2js.

tests/isolate/count_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 CountTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import 'dart:isolate';
 
 void countMessages() {
   int count = 0;
@@ skipping 12 matching lines @@
 
 void main() {
   test("count 10 consecutive messages", () {
     int count = 0;
     SendPort remote = spawnFunction(countMessages);
     ReceivePort local = new ReceivePort();
     SendPort reply = local.toSendPort();
 
     local.receive(expectAsync2((int message, SendPort replyTo) {
       if (message == -1) {
+        // [count] is '11' because when we sent '9' to [remote],
+        // the other isolate will send another message '18', that this
+        // isolate will receive. Then this isolate will send '10' to
+        // [remote] and increment [count]. Note that this last '10'
+        // message will not be received by the other isolate, since it
+        // received '-1' before.
         expect(count, 11);
         local.close();
         return;
       }
 
       expect(message, (count - 1) * 2);
       remote.send(count++, reply);
       if (count == 10) {
         remote.send(-1, reply);
       }
     }, 11));
     remote.send(count++, reply);
   });
 }