Fix a couple more tests related to Isolate.spawn semantics change

tests/language/vm/optimized_guarded_field_isolates_test.dart

 // Copyright (c) 2013, 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.
 // VMOptions=--optimization_counter_threshold=100
 
 // Test field type tracking and field list-length tracking in the presence of
 // multiple isolates.
 
 import "dart:isolate";
 import "dart:async";
 import "package:expect/expect.dart";
+import 'package:async_helper/async_helper.dart';
 
 class A {
   A(this.a);
   var a;
 }
 
 class B extends A {
   B(a, this.b) : super(a) { }
 
   var b;
 }
 
 f1(SendPort send_port) {
   send_port.send(new B("foo", "bar"));
 }
 
 test_b(B obj) => obj.a + obj.b;
 
 test_field_type() {
   var receive_port = new ReceivePort();
+  asyncStart();
   Future<Isolate> isolate = Isolate.spawn(f1, receive_port.sendPort);
   B b = new B(1, 2);
   for (var i = 0; i < 200; i++) { test_b(b); }
   Expect.equals(3, test_b(b));
   Future<B> item = receive_port.first;
   item.then((B value) {
       Expect.equals("foobar", test_b(value));
       receive_port.close();
+      asyncEnd();
   });
 }
 
 class C {
   C(this.list);
   final List list;
 }
 
 f2(SendPort send_port) {
   send_port.send(new C(new List(1)));
 }
 
 test_c(C obj) => obj.list[9999];
 
 test_list_length() {
   var receive_port = new ReceivePort();
+  asyncStart();
   Future<Isolate> isolate = Isolate.spawn(f2, receive_port.sendPort);
   C c = new C(new List(10000));
   for (var i = 0; i < 200; i++) { test_c(c); }
   Expect.equals(null, test_c(c));
   Future<C> item = receive_port.first;
   item.then((C value) {
       Expect.throws(() => test_c(value), (e) => e is RangeError);
       receive_port.close();
+      asyncEnd();
   });
 }
 
 main() {
   test_field_type();
   test_list_length();
 }