Support web workers in dart2js.

tests/isolate/isolate2_negative_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.
 
 // Dart test program for testing that exceptions in other isolates bring down
 // the program.
 
 library isolate2_negative_test;
 import 'dart:isolate';
 
 void entry() {
   throw "foo";
 }
 
 main() {
   SendPort port = spawnFunction(entry);
+  // TODO(ngeoffray): Remove this code and update dart2js to wait for
+  // all ports to be closed.
+  // Make sure the main thread stays alive so that the spawned isolate
+  // can run.
+  port.receive((msg) {});
 }