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

tests/language/vm/optimized_guarded_field_isolates_test.dart

Index: tests/language/vm/optimized_guarded_field_isolates_test.dart
diff --git a/tests/language/vm/optimized_guarded_field_isolates_test.dart b/tests/language/vm/optimized_guarded_field_isolates_test.dart
index fc5e915785151dec3a477a0f9a2d537b0faf66a6..a150656cf9e5ee5f600ec0e36c8d6294923e4658 100644
--- a/tests/language/vm/optimized_guarded_field_isolates_test.dart
+++ b/tests/language/vm/optimized_guarded_field_isolates_test.dart
@@ -9,6 +9,7 @@
 import "dart:isolate";
 import "dart:async";
 import "package:expect/expect.dart";
+import 'package:async_helper/async_helper.dart';
 
 class A {
   A(this.a);
@@ -29,6 +30,7 @@ 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); }
@@ -37,6 +39,7 @@ test_field_type() {
   item.then((B value) {
       Expect.equals("foobar", test_b(value));
       receive_port.close();
+      asyncEnd();
   });
 }
 
@@ -53,6 +56,7 @@ 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); }
@@ -61,6 +65,7 @@ test_list_length() {
   item.then((C value) {
       Expect.throws(() => test_c(value), (e) => e is RangeError);
       receive_port.close();
+      asyncEnd();
   });
 }