Added async_helper and fixed broken asynchronous test.

tests/isolate/isolate2_negative_test.dart

Index: tests/isolate/isolate2_negative_test.dart
diff --git a/tests/isolate/isolate2_negative_test.dart b/tests/isolate/isolate2_negative_test.dart
index 720b825ff327d61f28141b442a55d260115b4af0..01cf0ed29c6ebea7b278ddef121304918547a71f 100644
--- a/tests/isolate/isolate2_negative_test.dart
+++ b/tests/isolate/isolate2_negative_test.dart
@@ -7,11 +7,16 @@
 
 library isolate2_negative_test;
 import 'dart:isolate';
+import '../async_helper.dart';
 
 void entry() {
   throw "foo";
 }
 
 main() {
+  // We start an asynchronous operation, but since we don't expect to get
+  // anything back except an exception there is no asyncEnd().
+  // If the exception is not thrown this test will timeout.
+  asyncStart();
   SendPort port = spawnFunction(entry);
 }